Emerald

Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium. Beryl has a hardness of 7.5–8 on the Mohs scale. Most emeralds have many inclusions, so their toughness (resistance to breakage) is classified as generally poor. Emerald is a cyclosilicate.

Emerald

General

Category Beryl variety

Formula Be3Al2(SiO3)6

Crystal system Hexagonal (6/m 2/m 2/m) Space group: P6/mсc

Space group (6/m 2/m 2/m) – dihexagonal dipyramidal

Unit cell a = 9.21 Å, c = 9.19 Å; Z = 2

Identification

Formula mass 537.50

Color Bluish green to green

Crystal habit Massive to well Crystalline

Cleavage Imperfect on the 

Fracture Conchoidal

Mohs scale hardness 7.5–8

Luster Vitreous

Streak White

Diaphaneity Transparent to opaque

Specific gravity Average 2.76

Optical properties Uniaxial (−)

Refractive index nω = 1.564–1.595,

nε = 1.568–1.602

Birefringence δ = 0.0040–0.0070

Ultraviolet fluorescence None (some fracture-filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)

Etymology

The word "emerald" is derived (via Old French: esmeraude and Middle English: emeraude), from Vulgar Latin: esmaralda/esmaraldus, a variant of Latin smaragdus, which was via Ancient Greek: σμάραγδος (smáragdos; "green gem"). The Greek word may have a Semitic, Sanskrit or Persian origin. According to Webster's Dictionary the term emerald was first used in the 14th century.

Properties determining value

Emeralds, like all colored gemstones, are graded using four basic parameters known as "the four Cs": color, clarity, cut and carat weight. Normally, in grading colored gemstones, color is by far the most important criterion. However, in the grading of emeralds, clarity is considered a close second. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gemstone.

This member of the beryl family ranks among the traditional "big four" precious gems along with diamonds, rubies and sapphires.

In the 1960s, the American jewelry industry changed the definition of emerald to include the green vanadium-bearing beryl. As a result, vanadium emeralds purchased as emeralds in the United States are not recognized as such in the United Kingdom and Europe. In America, the distinction between traditional emeralds and the new vanadium kind is often reflected in the use of terms such as "Colombian emerald".

Color

In gemology, color is divided into three components: hue, saturation, and tone. Emeralds occur in hues ranging from yellow-green to blue-green, with the primary hue necessarily being green. Yellow and blue are the normal secondary hues found in emeralds. Only gems that are medium to dark in tone are considered emeralds; light-toned gems are known instead by the species name green beryl. The finest emeralds are approximately 75% tone on a scale where 0% tone is colorless and 100% is opaque black. In addition, a fine emerald will be saturated and have a hue that is bright (vivid). Gray is the normal saturation modifier or mask found in emeralds; a grayish-green hue is a dull-green hue.

Clarity

Emeralds tend to have numerous inclusions and surface-breaking fissures. Unlike diamonds, where the loupe standard (i.e., 10× magnification) is used to grade clarity, emeralds are graded by eye. Thus, if an emerald has no visible inclusions to the eye (assuming normal visual acuity) it is considered flawless. Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated to enhance the apparent clarity. The inclusions and fissures within an emerald are sometimes described as jardin (French for garden), because of their mossy appearance. Imperfections are unique for each emerald and can be used to identify a particular stone. Eye-clean stones of a vivid primary green hue (as described above), with no more than 15% of any secondary hue or combination (either blue or yellow) of a medium-dark tone, command the highest prices. The relative non-uniformity motivates the cutting of emeralds in cabochon form, rather than faceted shapes. Faceted emeralds are most commonly given an oval cut, or the signature emerald cut, a rectangular cut with facets around the top edge.

Treatments

Most emeralds are oiled as part of the post-lapidary process, in order to fill in surface-reaching cracks so that clarity and stability are improved. Cedar oil, having a similar refractive index, is often used in this widely adopted practice. Other liquids, including synthetic oils and polymers with refractive indexes close to that of emeralds, such as Opticon, are also used. The least expensive emeralds are often treated with epoxy resins, which are effective for filling stones with many fractures. These treatments are typically applied in a vacuum chamber under mild heat, to open the pores of the stone and allow the fracture-filling agent to be absorbed more effectively. The U.S. Federal Trade Commission requires the disclosure of this treatment when an oil-treated emerald is sold. The use of oil is traditional and largely accepted by the gem trade, although oil-treated emeralds are worth much less than untreated emeralds of similar quality. Untreated emeralds must also be accompanied by a certificate from a licensed, independent gemology laboratory. Other treatments, for example the use of green-tinted oil, are not acceptable in the trade. 

Gems are graded on a four-step scale; none, minor, moderate and highly enhanced. These categories reflect levels of enhancement, not clarity. A gem graded none on the enhancement scale may still exhibit visible inclusions. Laboratories apply these criteria differently. Some gemologists consider the mere presence of oil or polymers to constitute enhancement. Others may ignore traces of oil if the presence of the material does not improve the look of the gemstone.

Varieties

Inclusions in emeralds are normal and are called jardin inclusions. Unlike diamonds, inclusions do not reduce the price unless they appear disturbing. They serve more as a criterion for distinguishing them from synthetic stones. 

Trapiche emerald, a rare variety formed by inclusions and special growth processes in the shape of a wagon wheel with six spokes (no twins), mainly from Colombia

Emerald cat's eye, with cat's eye effect

Emerald mines

Emeralds in antiquity were mined in Ancient Egypt at locations on Mount Smaragdus since 1500 BC, and India and Austria since at least the 14th century AD. The Egyptian mines were exploited on an industrial scale by the Roman and Byzantine Empires, and later by Islamic conquerors. Mining in Egypt ceased with the discovery of the Colombian deposits. Today, only ruins remain in Egypt.

Colombia is by far the world's largest producer of emeralds, constituting 50–95% of the world production, with the number depending on the year, source and grade. Emerald production in Colombia has increased drastically in the last decade, increasing by 78% from 2000 to 2010. The three main emerald mining areas in Colombia are Muzo, Coscuez, and Chivor. Rare "trapiche" emeralds are found in Colombia, distinguished by ray-like spokes of dark impurities.

Zambia is the world's second biggest producer, with its Kafubu River area deposits (Kagem Mines) about 45 km (28 mi) southwest of Kitwe responsible for 20% of the world's production of gem-quality stones in 2004. In the first half of 2011, the Kagem Mines produced 3.74 tons of emeralds.

Emeralds are found all over the world in countries such as Afghanistan, Australia, Austria, Brazil, Bulgaria, Cambodia, Canada, China, Egypt, Ethiopia, France, Germany, India, Kazakhstan, Madagascar, Mozambique, Namibia, Nigeria, Norway, Pakistan, Russia, Somalia, South Africa, Spain, Switzerland, Tanzania, the United States, Zambia, and Zimbabwe. In the US, emeralds have been found in Connecticut, Montana, Nevada, North Carolina, and South Carolina. In 1998, emeralds were discovered in the Yukon Territory of Canada.

Origin determinations

Since the onset of concerns regarding diamond origins, research has been conducted to determine if the mining location could be determined for an emerald already in circulation. Traditional research used qualitative guidelines such as an emerald's color, style and quality of cutting, type of fracture filling, and the anthropological origins of the artifacts bearing the mineral to determine the emerald's mine location. More recent studies using energy-dispersive X-ray spectroscopy methods have uncovered trace chemical element differences between emeralds, including ones mined in close proximity to one another. American gemologist David Cronin and his colleagues have extensively examined the chemical signatures of emeralds resulting from fluid dynamics and subtle precipitation mechanisms, and their research demonstrated the chemical homogeneity of emeralds from the same mining location and the statistical differences that exist between emeralds from different mining locations, including those between the three locations: Muzo, Coscuez, and Chivor, in Colombia, South America.

Use as a gemstone

Emeralds have been and continue to be valued as particularly valuable jewelry and gemstones by many cultures around the world. Emeralds from Brazil, in particular, can sometimes fetch higher prices than a diamond of the same size due to their vibrant green color.

The preferred cut for clear emerald crystals is the emerald cut, an octagonal step cut that was developed with the stone's sensitivity to shock in mind. 

The most important cutting center is located in India, in Jaipur, where the number of lapidaries is estimated at 100,000. Emerald cutting developed there due to the Maharajah 's great predilection for precious stones, particularly emeralds from all over the world. Cutting is carried out with the aim of minimizing waste, as in Brazil. Stones cut in Colombia were cut according to the same principle until European cutting principles, favoring quality over yield, were adopted by the cutters of Bogota. The finest stones are generally recut in Paris. In addition, Israel has developed an ultra-modern cutting center in Ramat Gan.

Notable emeralds

Emerald	Origin	Size	Location
Chipembele	Zambia, 2021	7,525 carats (1.505 kg)	Israel Diamond Exchange, Eshed – Gemstar
Bahia Emerald	Brazil, 2001	180,000 carats, crystals in host rock 752 lb (341 kg)	Los Angeles County Sheriff's Department
Carolina Emperor	United States, 2009	310 carats uncut, 64.8 carats cut	North Carolina Museum of Natural Sciences, Raleigh
Chalk Emerald	Colombia	38.40 carats cut, then recut to 37.82 carats	National Museum of Natural History, Washington
Duke of Devonshire Emerald	Colombia, before 1831	1,383.93 carats uncut	Natural History Museum, London
Emerald of Saint Louis	Austria, probably Habachtal	51.60 carats cut	National Museum of Natural History, Paris
Gachalá Emerald	Colombia, 1967	858 carats uncut	National Museum of Natural History, Washington
Mogul Mughal Emerald	Colombia, 1107 A.H. (1695–1696 AD)	217.80 carats cut	Museum of Islamic Art, Doha, Qatar
Rockefeller Emerald	Colombia	18.04 carats Octagonal step-cut	Private collection
Patricia Emerald	Colombia, 1920	632 carats uncut, dihexagonal (12 sided)	American Museum of Natural History, New York
Mim Emerald	Colombia, 2014	1,390 carats uncut, dihexagonal (12 sided)	Mim Museum, Beirut

Synthetic emerald

Both hydrothermal and flux-growth synthetics have been produced, and a method has been developed for producing an emerald overgrowth on colorless beryl. The first commercially successful emerald synthesis process was that of Carroll Chatham, likely involving a lithium vanadate flux process, as Chatham's emeralds do not have any water and contain traces of vanadate, molybdenum and vanadium. The other large producer of flux emeralds was Pierre Gilson Sr., whose products have been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds, which are coated on both sides. Growth occurs at the rate of 1 mm per month, a typical seven-month growth run produces emerald crystals 7 mm thick.

Hydrothermal synthetic emeralds have been attributed to IG Farben, Nacken, Tairus, and others, but the first satisfactory commercial product was that of Johann Lechleitner of Innsbruck, Austria, which appeared on the market in the 1960s. These stones were initially sold under the names "Emerita" and "Symeralds", and they were grown as a thin layer of emerald on top of natural colorless beryl stones. Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents (attributable to E.M. Flanigen), acidic conditions are essential to prevent the chromium (which is used as the colorant) from precipitating. Also, it is important that the silicon-containing nutrient be kept away from the other ingredients to prevent nucleation and confine growth to the seed crystals. Growth occurs by a diffusion-reaction process, assisted by convection. The largest producer of hydrothermal emeralds today is Tairus, which has succeeded in synthesizing emeralds with chemical composition similar to emeralds in alkaline deposits in Colombia, and whose products are thus known as “Colombian created emeralds” or “Tairus created emeralds”. Luminescence in ultraviolet light is considered a supplementary test when making a natural versus synthetic determination, as many, but not all, natural emeralds are inert to ultraviolet light. Many synthetics are also UV inert.

Synthetic emeralds are often referred to as "created", as their chemical and gemological composition is the same as their natural counterparts. The U.S. Federal Trade Commission (FTC) has very strict regulations as to what can and what cannot be called a "synthetic" stone. The FTC says: "§ 23.23(c) It is unfair or deceptive to use the word "laboratory-grown", "laboratory-created", "[manufacturer name]-created", or "synthetic" with the name of any natural stone to describe any industry product unless such industry product has essentially the same optical, physical, and chemical properties as the stone named."

Manipulations and imitations

Although emerald is very resistant to heat in terms of its optical properties (color, luster), it only changes color at temperatures of around 700 to 800 °C. However, it often exhibits uneven color distribution and, due to numerous cracks and inclusions, color clouding and sensitivity to pressure. This is counteracted in the jewelry industry by stabilizing the stone with, for example, uncolored synthetic resins or coloring it with colored oils and resins.

As one of the most valuable and correspondingly expensive gemstones, emerald is often supplemented or replaced with synthetics and imitations. Color-similar minerals such as green-colored minerals of the tourmaline group, diopside, dioptase, and the garnet minerals and varieties grossular and demantoid are used, as is colored glass.

Historical and cultural references

Emerald is regarded as the traditional birthstone for May as well as the traditional gemstone for the astrological sign of Taurus.

Traditional alchemical lore ascribes several uses and characteristics to emeralds:

The virtue of the Emerald is to counteract poison. They say that if a venomous animal should look at it, it will become blinded. The gem also acts as a preservative against epilepsy; it cures leprosy, strengthens sight and memory, checks copulation, during which act it will break, if worn at the time on the finger.

According to French writer Brantôme (c. 1540–1614) Hernán Cortés had one of the emeralds which he had looted from Mexico text engraved, Inter Natos Mulierum non surrexit major ("Among those born of woman there hath not arisen a greater," Matthew 11:11), in reference to John the Baptist. Brantôme considered engraving such a beautiful and simple product of nature sacrilegious and considered this act the cause for Cortez's loss in 1541 of an extremely precious pearl, and even for the death of King Charles IX of France, who died (1574) soon afterward.

In American author L. Frank Baum's 1900 children's novel The Wonderful Wizard of Oz, and the 1939 MGM film adaptation, the protagonist must travel to an Emerald City to meet the eponymous character, the Wizard.

The chief deity of one of India's most famous temples, the Meenakshi Amman Temple in Madurai, is the goddess Meenakshi, whose idol is traditionally thought to be made of emerald.

Symbolism

In the Middle Ages, it was a symbol of John the Apostle. For alchemists, it is the stone of Venus.

Superstition, since ancient times, has attributed miraculous virtues to this stone, such as preventing the symptoms of epilepsy and breaking when the disease had reached such a state of violence that she could not overcome it; and accelerating labor when tied to the thigh of a woman about to give birth. Finally, emerald powder cured dysentery and the bites of poisonous animals.

The people of the Manta Valley in Ecuador worshipped an emerald the size of an ostrich egg; they would display it on major festival days, and the Indians would come running from all over to see their goddess and offer her other emeralds. The priests and chieftains would make the believers understand that the mother emerald was very happy because they were presenting her daughters to her, thus managing to gather a large quantity of precious stones. When the Spanish expeditionaries conquered the region, they are said to have found all the daughters of the goddess, but the Indians were so good at hiding the mother that her whereabouts have still not been discovered. 

In pop culture

The emerald, or rather its green color, is frequently mentioned in literature. For example, a 2010 novel by Kerstin Gier from the Love Goes Through All Times series is titled Emerald Green. A German film adaptation of the same name was released in 2016. 

The comic book superhero Green Lantern is also known as the Emerald Knight due to his green costume.  

In the computer game Minecraft, emerald plays a significant role, among other things, as a currency in trade with villagers. 

Sourced from Wikipedia

Fluorite

Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.

The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 4 as fluorite.

Pure fluorite is colourless and transparent, both in visible and ultraviolet light, but impurities usually make it a colorful mineral and the stone has ornamental and lapidary uses. Industrially, fluorite is used as a flux for smelting, and in the production of certain glasses and enamels. The purest grades of fluorite are a source of fluoride for hydrofluoric acid manufacture, which is the intermediate source of most fluorine-containing fine chemicals. Optically clear transparent fluorite has anomalous partial dispersion, that is, its refractive index varies with the wavelength of light in a manner that differs from that of commonly used glasses, so fluorite is useful in making apochromatic lenses, and particularly valuable in photographic optics. Fluorite optics are also usable in the far-ultraviolet and mid-infrared ranges, where conventional glasses are too opaque for use. Fluorite also has low dispersion, and a high refractive index for its density.

Fluorite

General

Category Halide mineral

Formula CaF2

IMA symbol Flr

Strunz classification 3.AB.25

Crystal system Isometric

Crystal class Hexoctahedral (m3m)

H–M symbol: (4/m 3 2/m)

(cF12)

Space group Fm3m (No. 225)

Unit cell a = 5.4626 Å; Z = 4

Identification

Color Colorless, although samples are often deeply colored owing to impurities; Purple, lilac, golden-yellow, green, blue, pink, champagne, brown.

Crystal habit Well-formed coarse sized crystals; also nodular, botryoidal, rarely columnar or fibrous; granular, massive

Twinning Common on {111}, interpenetrant, flattened

Cleavage Octahedral, perfect on {111}, parting on {011}

Fracture Subconchoidal to uneven

Tenacity Brittle

Mohs scale hardness 4 (defining mineral)

Luster Vitreous

Streak White

Diaphaneity Transparent to translucent

Specific gravity 3.175–3.184; to 3.56 if high in rare-earth elements

Optical properties Isotropic; weak anomalous anisotropism; moderate relief

Refractive index 1.433–1.448

Fusibility 3

Solubility slightly water soluble and in hot hydrochloric acid

Other characteristics May be fluorescent, phosphorescent, thermoluminescent, and/or triboluminescent

History and etymology

The word fluorite is derived from the Latin verb fluere, meaning to flow. The mineral is used as a flux in iron smelting to decrease the viscosity of slag. The term flux comes from the Latin adjective fluxus, meaning flowing, loose, slack. The mineral fluorite was originally termed fluorspar and was first discussed in print in a 1530 work Bermannvs sive de re metallica dialogus [Bermannus; or dialogue about the nature of metals], by Georgius Agricola, as a mineral noted for its usefulness as a flux. Agricola, a German scientist with expertise in philology, mining, and metallurgy, named fluorspar as a Neo-Latinization of the German Flussspat from Fluss (stream, river) and Spat (meaning a nonmetallic mineral akin to gypsum, spærstān, spear stone, referring to its crystalline projections).

In 1852, fluorite gave its name to the phenomenon of fluorescence, which is prominent in fluorites from certain locations, due to certain impurities in the crystal. Fluorite also gave the name to its constitutive element fluorine. Currently, the word "fluorspar" is most commonly used for fluorite as an industrial and chemical commodity, while "fluorite" is used mineralogically and in most other senses.

In archeology, gemmology, classical studies, and Egyptology, the Latin terms murrina and myrrhina refer to fluorite. In book 37 of his Naturalis Historia, Pliny the Elder describes it as a precious stone with purple and white mottling, and noted that the Romans prized objects carved from it. It has been suggested that the Sanskrit mineral name vaikrānta (वैक्रान्तः), known from Sanskrit alchemical texts dating from the early second millennium CE onwards, may refer to fluorite.

Classification

In the outdated, but still partly used 8th edition of the mineral systematics according to Strunz, fluorite belonged to the general division of the "simple halides", where it formed a separate group together with coccinite, frankdicksonite, gagarinite-(Y), laurelite, tveitite-(Y) and gagarinite-(Ce) (formerly zajacite-(Ce)).

The 9th edition of Strunz's mineral classification, valid since 2001 and used by the International Mineralogical Association (IMA), classifies fluorite in the new and more precise division of "simple halides without H 2 O". This division is further subdivided according to the molar ratio of cations (M) to anions (X), so that the mineral can be found in the subdivision "M: X = 1: 2" according to its composition, where it gives its name to the "fluorite group" with the system number 3.AB.25 and the other members fluorocronite (IMA 2010-023), frankdicksonite, and strontiofluorite (IMA 2009-014).

The Dana classification of minerals commonly used in English-speaking countries classifies fluorite in the class of "halides (and related)" and within the division of "halides." Here, it is the eponymous mineral of the "fluorite group" with the system number 09.02.01, along with the other members frankdicksonite, tveitite-(Y), and strontiofluorite within the subdivision of " anhydrous and hydrous halides with the formula AX 2 ".

Crystal structure

Fluorite crystallizes in the cubic crystal system in the highly symmetric crystal class 4/ m 3 2/ m (cubic-hexakisoctahedral) or the space group Fm 3 m (space group no. 225) with the lattice parameter a = 5.463 Å and 4 formula units per unit cell. 

Fluorite crystallizes in a cubic motif. Crystal twinning is common and adds complexity to the observed crystal habits. Fluorite has four perfect cleavage planes that help produce octahedral fragments. The structural motif adopted by fluorite is so common that the motif is called the fluorite structure. Element substitution for the calcium cation often includes strontium and certain rare-earth elements (REE), such as yttrium and cerium.

The crystal structure of fluorite was elucidated in 1914 by William Henry Bragg and his son William Lawrence Bragg using X-ray diffraction experiments. Ca 2+ ions form a cubic closest packing, which corresponds to a face-centered cubic lattice (fcc). The face-centered position of the unit cell can also be read from the space group symbol ("F"). The fluoride ions (F −) occupy all tetrahedral holes in the closest packing of calcium ions. Since there are always twice as many tetrahedral holes as packing particles in a closest packing, the structure has a calcium to fluorine ratio of 1:2, which is also reflected in the chemical formula of fluorite, CaF 2. The fluoride ions therefore form a tetrahedron of four calcium ions, which are surrounded by eight fluorine atoms in the shape of a cube. The cation and anion sub-lattices are non-commutative, i.e., they are interchangeable. The so-called fluorite structure is found in a number of other salts, such as the fluorides SrF 2, BaF 2, CdF 2, HgF 2, and PbF 2. The fluorite structure also occurs, for example, in Li 2 O, Li 2 S, Na 2 O, Na 2 S, K 2 O, K 2 S, Rb 2 O, and Rb 2 S. Its crystal structure is isotypic with uraninite

Characteristics

Morphology

Fluorite frequently forms well-formed, cubic and octahedral crystals. In combination with these main forms, fluorite crystals often exhibit faces of other shapes. Common are the faces of the rhombic dodecahedron {110}, the tetrakis hexahedron {210} (additional faces parallel to the cube edges), the icositetrahedron {211} or {311}, and the hexakis octahedron {421}.

The crystal structure of fluorite crystals is temperature-dependent. Thus, at high formation temperatures, octahedra {111} are predominantly formed, at medium temperatures, rhombic dodecahedra {110} are more likely, and at low temperatures, cubes {100} are the dominant structures. 

The cube faces are usually smooth and shiny. Octahedral and rhombic dodecahedral faces, on the other hand, often appear rough and matte and are usually composed of tiny cube faces. The loose, octahedral fluorite crystals with smooth, shiny faces commonly found in the trade are almost never crystals grown in this shape, but rather cleavage octahedra.

Fluorite also forms spherical and grape-shaped aggregates, crusts, and even stalactites. Scalenohedral fluorites are a special case, as evidenced by finds from the Cäcilia fluorspar mine near Freiung (Stulln municipality in Upper Palatinate). However, closer examination revealed that these also belong to the cubic system and were formed solely through etching. 

Physical properties

By incorporating lanthanides, for example Eu 2+, fluorite can exhibit strong fluorescence when excited by UV light and also phosphorescence when heated and triboluminescence when subjected to strong mechanical stress.

Fluorite is an electrical non-conductor. Its melting point is 1392 °C. 

In thin sections under the microscope, fluorite is striking in linearly polarized light because, due to its relatively low refractive index, it exhibits a strongly negative relief compared to almost all accompanying minerals. Under crossed polarizing filters, it exhibits isotropy as a cubically crystallized mineral, meaning it remains dark. 

Chemical properties

When in contact with strong acids such as sulfuric acid, fluorite releases highly toxic hydrogen fluoride.

Fluorescence

George Gabriel Stokes named the phenomenon of fluorescence from fluorite, in 1852.

Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite. Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green, and white also occur. The fluorescence of fluorite may be due to mineral impurities, such as yttrium and ytterbium, or organic matter, such as volatile hydrocarbons in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal. Natural samples containing rare earth impurities such as erbium have also been observed to display upconversion fluorescence, in which infrared light stimulates emission of visible light, a phenomenon usually only reported in synthetic materials.

One fluorescent variety of fluorite is chlorophane, which is reddish or purple in color and fluoresces brightly in emerald green when heated (thermoluminescence), or when illuminated with ultraviolet light.

The color of visible light emitted when a sample of fluorite is fluorescing depends on where the original specimen was collected; different impurities having been included in the crystal lattice in different places. Neither does all fluorite fluoresce equally brightly, even from the same locality. Therefore, ultraviolet light is not a reliable tool for the identification of specimens, nor for quantifying the mineral in mixtures. For example, among British fluorites, those from Northumberland, County Durham, and eastern Cumbria are the most consistently fluorescent, whereas fluorite from Yorkshire, Derbyshire, and Cornwall, if they fluoresce at all, are generally only feebly fluorescent.

Fluorite also exhibits the property of thermoluminescence.

Color

Although pure CaF2 is colorless, fluorite is one of the minerals with the most color variation. The dark color of many fluorites is caused by embedded rare earth elements or radioactive irradiation of the fluorspar (smelly spar), although intergrown uranium minerals can also enhance the color.

Fluorite is allochromatic, meaning that it can be tinted with elemental impurities. Fluorite comes in a wide range of colors and has consequently been dubbed "the most colorful mineral in the world". Every color of the rainbow in various shades is represented by fluorite samples, along with white, black, and clear crystals. The most common colors are purple, blue, green, yellow, or colorless. Less common are pink, red, white, brown, and black. Color zoning or banding is commonly present. The color of the fluorite is determined by factors including impurities, exposure to radiation, and the absence of voids of the color centers.

The causes of color are diverse and not always fully understood. The coloring is usually caused by trace amounts of rare earth elements, which are often only ionized into coloring ions through radioactive irradiation. Which rare earth elements are ionized can depend on the type of irradiation. For example, fluorites with the same trace element contents can develop different colors in the vicinity of thorium-containing minerals than in the vicinity of uranium-containing minerals. Furthermore, the temperature history can influence the color, as can the incorporation of oxygen ions and OH − or other coloring ions. Trace amounts of non-coloring ions such as Na + or Fe 3+ stabilize coloring lattice defects and thus also influence the color. 

Yellow: The color of yellow fluorites is due to the incorporation of O 3 − and O 2 − in place of two adjacent F − ions. Charge balancing occurs through the replacement of Ca 2+ by Na +. 

Light green: The light green color of many fluorites is due to trace amounts of Sm 2+. Samarium (Sm) is incorporated as Sm 3+ instead of Ca 2+. Reduction to Sm 2+ occurs by the absorption of an electron released during the oxidation of other cations by ionizing radiation. 

The stability of the green color depends on which cations are oxidized. Fluorites formed under reducing conditions contain traces of Fe 2+, which can be oxidized to Fe 3+ and, together with samarium, produces a temperature-stable green color. In fluorites formed under more oxidizing conditions, the generation and stabilization of the green-coloring Sm 2+ ions occurs by oxidation of Ce(Pr, Tb) 3+ to Ce(Pr, Tb) 4+. Fluorites colored by this mechanism bleach in daylight or upon heating. 

Yellow-green: Fluorites from some localities (e.g., Redruth in England) exhibit a green to yellow-green coloration, which is due to the co-occurrence of traces of yttrium (Y 3+) and cerium (Ce 3+), each with a color center (vacancy at an F − position containing two electrons) in close proximity. 

Light blue: Fluorites containing only Y 3+, together with a vacancy at an F − position containing two electrons, are colored light blue. 

Dark blue: Synthetic fluorites can be colored intensely blue to violet by the formation of colloidal calcium (metallic). The blue-violet color of the naturally occurring "Blue John Fluorite" from Castelton near Derbyshire in England is also attributed to this. 

Violet: The cause of the widespread violet coloration of natural fluorite is not fully understood. Currently, electron defects in the crystal lattice are considered the most likely cause of the violet color of fluorite. 

Pink, red: The pink to red color of fluorites is caused by the incorporation of O 2 3− molecules, which are stabilized by neighboring Y 3+ ions. 

Stinkspar (antozonite)

Stink spar is a dark purple to black variety of fluorite that develops a pungent odor when crushed. Stink spar often (but not always) occurs together with uranium minerals, some of which may be enclosed in the spar as very fine particles. The type locality and best-known German site is Wölsendorf in the Upper Palatinate.

Rubbing or striking the crystal releases gaseous toxic fluorine (F 2), which causes the odor.

The dark purple to black color has several causes. Colloidal metallic calcium plays a major role, resulting in a dark blue to black color. In addition, there are free electrons on empty fluorine positions (F centers), which are typical of violet fluorite. 

All of these properties of fluorite are due to radioactive irradiation of the fluorite. Fluorite typically occurs together with uranium-bearing minerals. The uranium and thorium they contain decay, emitting gamma radiation. This radiation releases electrons from the F− ions, forming an H center: a neutral fluorine atom in an otherwise empty lattice position that forms an atomic bond with a neighboring F− ion. The released electrons are captured by lattice vacancies, empty fluorine positions, where they form F centers: single electrons in an F− position surrounded by four Ca2 + ions. These F centers are not spatially stable. They diffuse through the crystal lattice and combine with other F centers to form fluorine-free Ca nanoparticles with a diameter of 5 to 30 nm. These clusters are also called “colloidal Ca” and contribute to the blue-black color of the smelt. 

Varieties

To date, several varieties of fluorite are known in which small amounts of calcium are replaced by rare earths such as cerium and yttrium:

Yttriumfluorite (1911) and cerium fluorite were first described by Thorolf Vogt as new mineral species from northern Norway. After further melt-flow analyses in 1913 by Gustav Tammann and Vogt, it was determined that fluorite can contain up to 50% by weight yttrium fluoride (YF 3, yttrofluorite) and up to 55.8% cerium(III) fluoride (CeF 3, cerium fluorite). Therefore, a three-component solid solution system theoretically exists, although the idealized compositions YF 3 and CeF 3 have so far only been known synthetically and have only been investigated up to the above-mentioned percentages of fluorite. 

Neither cerium fluorite nor yttrium fluorite have so far been discovered in nature in a material purity high enough to be recognized by the International Mineralogical Association (IMA). Yttrofluorite was officially discredited by the IMA in 2006, while cerium fluorite remained listed as a hypothetical mineral in the IMA mineral list until 2009. 

Yttrocerite was described as a mineral in 1815 by Johan Gottlieb Gahn and Jöns Jakob Berzelius; However, in 1913, Vogt stated in his analytical results that it was a mixture of yttrofluorite and cerium fluorite. 

Occurrence and mining

Fluorite forms as a late-crystallizing mineral in felsic igneous rocks typically through hydrothermal activity. It is particularly common in granitic pegmatites. It may occur as a vein deposit formed through hydrothermal activity particularly in limestones. In such vein deposits it can be associated with galena, sphalerite, barite, quartz, and calcite. Fluorite can also be found as a constituent of sedimentary rocks either as grains or as the cementing material in sandstone.

It is a common mineral mainly distributed in South Africa, China, Mexico, Mongolia, the United Kingdom, the United States, Canada, Tanzania, Rwanda and Argentina.

The world reserves of fluorite are estimated at 230 million tonnes (Mt) with the largest deposits being in South Africa (about 41 Mt), Mexico (32 Mt) and China (24 Mt). China is leading the world production with about 3 Mt annually (in 2010), followed by Mexico (1.0 Mt), Mongolia (0.45 Mt), Russia (0.22 Mt), South Africa (0.13 Mt), Spain (0.12 Mt) and Namibia (0.11 Mt).

One of the largest deposits of fluorspar in North America is located on the Burin Peninsula, Newfoundland, Canada. The first official recognition of fluorspar in the area was recorded by geologist J.B. Jukes in 1843. He noted an occurrence of "galena" or lead ore and fluoride of lime on the west side of St. Lawrence harbour. It is recorded that interest in the commercial mining of fluorspar began in 1928 with the first ore being extracted in 1933. Eventually, at Iron Springs Mine, the shafts reached depths of 970 feet (300 m). In the St. Lawrence area, the veins are persistent for great lengths and several of them have wide lenses. The area with veins of known workable size comprises about 60 square miles (160 km2).

In 2018, Canada Fluorspar Inc. commenced mine production again in St. Lawrence; in spring 2019, the company was planned to develop a new shipping port on the west side of Burin Peninsula as a more affordable means of moving their product to markets, and they successfully sent the first shipload of ore from the new port on July 31, 2021. This marks the first time in 30 years that ore has been shipped directly out of St. Lawrence.

Cubic crystals up to 20 cm across have been found at Dalnegorsk, Russia. The largest documented single crystal of fluorite was a cube 2.12 meters in size and weighing approximately 16 tonnes.

In Asturias (Spain) there are several fluorite deposits known internationally for the quality of the specimens they have yielded. In the area of Berbes, Ribadesella, fluorite appears as cubic crystals, sometimes with dodecahedron modifications, which can reach a size of up to 10 cm of edge, with internal colour zoning, almost always violet in colour. It is associated with quartz and leafy aggregates of baryte. In the Emilio mine, in Loroñe, Colunga, the fluorite crystals, cubes with small modifications of other figures, are colourless and transparent. They can reach 10 cm of edge. In the Moscona mine, in Villabona, the fluorite crystals, cubic without modifications of other shapes, are yellow, up to 3 cm of edge. They are associated with large crystals of calcite and barite.

"Blue John"

One of the most famous of the older-known localities of fluorite is Castleton in Derbyshire, England, where, under the name of "Derbyshire Blue John", purple-blue fluorite was extracted from several mines or caves. During the 19th century, this attractive fluorite was mined for its ornamental value. The mineral Blue John is now scarce, and only a few hundred kilograms are mined each year for ornamental and lapidary use. Mining still takes place in Blue John Cavern and Treak Cliff Cavern.

Recently discovered deposits in China have produced fluorite with coloring and banding similar to the classic Blue John stone.

Uses

As a raw material

Fluorite is mainly used industrially

as metallurgical spar in the metal industry as a flux for slag in the iron and steel process, especially as an additive in the Siemens-Martin furnace and in the electric arc furnace, and for the production of artificial cryolite for the production of aluminium,

as acid spar for the production of fluorine and hydrofluoric acid as well as various fluorides and derivatives such as fluorocarbons and polymeric fluorine compounds (e.g. polytetrafluoroethylene),

as ceramic spar in the glass industry as a flux and opacifier for e.g. milk glass, frosted glass and opalescent glasses, for ceramic materials and as a basic material for optical lenses (CaF2 single crystals, fluoride glasses based on beryllium fluoride, fluorite and sodium fluoride). Due to the property of refracting the light spectrum evenly, the chromatic aberration of lenses can be compensated. The problem here is that particularly large crystals are needed for high-performance lenses, and these are grown artificially. Crystals of this size have the property of warping due to heat (from sunlight) to such an extent that they significantly change the optical calculations.

Source of fluorine and fluoride

Fluorite is a major source of hydrogen fluoride, a commodity chemical used to produce a wide range of materials. Hydrogen fluoride is liberated from the mineral by the action of concentrated sulfuric acid:

CaF2(s) + H2SO4 → CaSO4(s) + 2 HF(g)

The resulting HF is converted into fluorine, fluorocarbons, and diverse fluoride materials. As of the late 1990s, five billion kilograms were mined annually.

There are three principal types of industrial use for natural fluorite, commonly referred to as "fluorspar" in these industries, corresponding to different grades of purity. Metallurgical grade fluorite (60–85% CaF2), the lowest of the three grades, has traditionally been used as a flux to lower the melting point of raw materials in steel production to aid the removal of impurities, and later in the production of aluminium. Ceramic grade fluorite (85–95% CaF2) is used in the manufacture of opalescent glass, enamels, and cooking utensils. The highest grade, "acid grade fluorite" (97% or more CaF2), accounts for about 95% of fluorite consumption in the US where it is used to make hydrogen fluoride and hydrofluoric acid by reacting the fluorite with sulfuric acid.

Internationally, acid-grade fluorite is also used in the production of AlF3 and cryolite (Na3AlF6), which are the main fluorine compounds used in aluminium smelting. Alumina is dissolved in a bath that consists primarily of molten Na3AlF6, AlF3, and fluorite (CaF2) to allow electrolytic recovery of aluminium. Fluorine losses are replaced entirely by the addition of AlF3, the majority of which react with excess sodium from the alumina to form Na3AlF6.

Uses as a gemstone

Natural fluorite mineral has ornamental and lapidary uses. Fluorite may be drilled into beads and used in jewelry, although due to its relative softness it is not widely used as a semiprecious stone. It is also used for ornamental carvings, with expert carvings taking advantage of the stone's zonation.

Due to its relatively low hardness and perfect cleavage, fluorite is of little interest as a gemstone for the commercial jewelry industry. Occasionally, it is processed by glyptics and hobby cutters into small, handcrafted objects or faceted gemstones. However, because of its wide range of colors, it can be confused with many gemstone minerals, and is often used as a basis for imitations. To change the colors, fluorite is either fired or irradiated. To protect against damage or to cover cracks, fluorite gemstones are often stabilized with synthetic resin.

Optics

In the laboratory, calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 150 to 9000 nm) and exhibits an extremely low change in refractive index with wavelength. Furthermore, the material is attacked by few reagents. At wavelengths as short as 157 nm, a common wavelength used for semiconductor stepper manufacture for integrated circuit lithography, the refractive index of calcium fluoride shows some non-linearity at high power densities, which has inhibited its use for this purpose. In the early years of the 21st century, the stepper market for calcium fluoride collapsed, and many large manufacturing facilities have been closed. Canon and other manufacturers have used synthetically grown crystals of calcium fluoride components in lenses to aid apochromatic design, and to reduce light dispersion. This use has largely been superseded by newer glasses and computer-aided design. As an infrared optical material, calcium fluoride is widely available and was sometimes known by the Eastman Kodak trademarked name "Irtran-3", although this designation is obsolete.

Fluorite should not be confused with fluoro-crown (or fluorine crown) glass, a type of low-dispersion glass that has special optical properties approaching fluorite. True fluorite is not a glass but a crystalline material. Lenses or optical groups made using this low dispersion glass as one or more elements exhibit less chromatic aberration than those utilizing conventional, less expensive crown glass and flint glass elements to make an achromatic lens. Optical groups employ a combination of different types of glass; each type of glass refracts light in a different way. By using combinations of different types of glass, lens manufacturers are able to cancel out or significantly reduce unwanted characteristics; chromatic aberration being the most important. The best of such lens designs are often called apochromatic. Fluoro-crown glass (such as Schott FK51) usually in combination with an appropriate "flint" glass (such as Schott KzFSN 2) can give very high performance in telescope objective lenses, as well as microscope objectives, and camera telephoto lenses. Fluorite elements are similarly paired with complementary "flint" elements (such as Schott LaK 10). The refractive qualities of fluorite and of certain flint elements provide a lower and more uniform dispersion across the spectrum of visible light, thereby keeping colors focused more closely together. Lenses made with fluorite are superior to fluoro-crown based lenses, at least for doublet telescope objectives; but are more difficult to produce and more costly.

The use of fluorite for prisms and lenses was studied and promoted by Victor Schumann near the end of the 19th century. Naturally occurring fluorite crystals without optical defects were only large enough to produce microscope objectives.

With the advent of synthetically grown fluorite crystals in the 1950s - 60s, it could be used instead of glass in some high-performance optical telescope and camera lens elements. In telescopes, fluorite elements allow high-resolution images of astronomical objects at high magnifications. Canon Inc. produces synthetic fluorite crystals that are used in their better telephoto lenses. The use of fluorite for telescope lenses has declined since the 1990s, as newer designs using fluoro-crown glass, including triplets, have offered comparable performance at lower prices. Fluorite and various combinations of fluoride compounds can be made into synthetic crystals which have applications in lasers and special optics for UV and infrared.

Exposure tools for the semiconductor industry make use of fluorite optical elements for ultraviolet light at wavelengths of about 157 nanometers. Fluorite has a uniquely high transparency at this wavelength. Fluorite objective lenses are manufactured by the larger microscope firms (Nikon, Olympus, Carl Zeiss and Leica). Their transparence to ultraviolet light enables them to be used for fluorescence microscopy. The fluorite also serves to correct optical aberrations in these lenses. Nikon has previously manufactured at least one fluorite and synthetic quartz element camera lens (105 mm f/4.5 UV) for the production of ultraviolet images. Konica produced a fluorite lens for their SLR cameras – the Hexanon 300 mm f/6.3.

Source of fluorine gas in nature

In 2012, the first source of naturally occurring fluorine gas was found in fluorite mines in Bavaria, Germany. It was previously thought that fluorine gas did not occur naturally because it is so reactive, and would rapidly react with other chemicals. Fluorite is normally colorless, but some varied forms found nearby look black, and are known as 'fetid fluorite' or antozonite. The minerals, containing small amounts of uranium and its daughter products, release radiation sufficiently energetic to induce oxidation of fluoride anions within the structure, to fluorine that becomes trapped inside the mineral. The color of fetid fluorite is predominantly due to the calcium atoms remaining. Solid-state fluorine-19 NMR carried out on the gas contained in the antozonite, revealed a peak at 425 ppm, which is consistent with F2.

Sourced from Wikipedia

Garnet

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, defining distinct species. These species fall into two primary solid solution series: the pyralspite series (pyrope, almandine, spessartine), with the general formula [Mg,Fe,Mn]3Al2(SiO4)3; and the ugrandite series (uvarovite, grossular, andradite), with the general formula Ca3[Cr,Al,Fe]2(SiO4)3. Notable varieties of grossular include hessonite and tsavorite.

Garnet

General

Category Nesosilicate

Formula The general formula X3Y2(SiO4)3

IMA symbol Grt

Crystal system Isometric

Crystal class

Space group Ia3d

Identification

Color virtually all colors, blue is rare

Crystal habit Rhombic dodecahedron or cubic

Cleavage Indistinct

Fracture conchoidal to uneven

Mohs scale hardness 6.5–7.5

Luster vitreous to resinous

Streak White

Diaphaneity Can form with any diaphaneity, translucent is common

Specific gravity 3.1–4.3

Polish luster vitreous to subadamantine

Optical properties Single refractive, often anomalous double refractive

Refractive index 1.72–1.94

Birefringence None

Pleochroism None

Ultraviolet fluorescence variable

Other characteristics variable magnetic attraction

Major varieties

Pyrope Mg3Al2Si3O12

Almandine Fe3Al2Si3O12

Spessartine Mn3Al2Si3O12

Andradite Ca3Fe2Si3O12

Grossular Ca3Al2Si3O12

Uvarovite Ca3Cr2Si3O12

Etymology

The word garnet comes from the 14th-century Middle English word gernet, meaning 'dark red'. It is borrowed from Old French grenate from Latin granatus, from granum ('grain, seed'). This is possibly a reference to mela granatum or even pomum granatum ('pomegranate', Punica granatum), a plant whose fruits contain abundant and vivid red seed covers (arils), which are similar in shape, size, and color to some garnet crystals. Hessonite garnet is also named 'gomed' in Indian literature and is one of the nine jewels in Vedic astrology that comprise the Navaratna.

Classification

Strunz

Already in the outdated 8th edition of the mineral systematics according to Strunz, the garnet group belonged to the general division of the " Island silicates (nesosilicates)", bears the system number VIII/A.08 and consisted of the members almandine, andradite, calderite, goldmanite, grossular, henriterite, hibschite, holtstamite, hydrougrandite (discredited in 1967 as an unnecessary group name), katoite, kimzeyite, knorringite, majorite, morimotoite, pyrope, schorlomite, spessartine, uvarovite, wadalite and yamatoite (discredited because identical with momoiite).

The 9th edition of Strunz's mineral classification, last updated in 2009, also classifies the garnet group in the division of "island silicates". However, this division is further subdivided according to the possible presence of other anions and the coordination of the cations involved, so that the garnet group with the system number 9.AD.25, according to the composition of the members almandine, andradite, blythite, calderite, goldmanite, grossular, henrittermite, hibschite, holtstamite, hydroandradite, katoite, kimzeyite, knorringite, majorite, momoiite (IMA 2009-026), morimotoite, pyrope, schorlomite, spessartine, skiagite, uvarovite and wadalite, can be found in the subdivision "island silicates without other anions; cations in octahedral and usually larger coordination".

Dana

The Dana mineral classification system, which is predominantly used in English-speaking countries, also classifies the garnet group under the “island silicate minerals” category. However, it is divided into the subgroups “pyralpite series” (system no. 51.04.03a), “ugrandite series” (system no. 51.04.03b), “schorlomite-kimzeyite series” (system no. 51.04.03c), “hydrogarnets” (system no. 51.04.03d), and “tetragonal hydrogarnets” (system no. 51.04.04) within the subgroup “ island silicates: SiO4 groups only with cations in and > coordination.”

IMA/CNMNC

The classical classifications listed above define the supergroups (classes) based on their composition and subdivide them according to structural criteria.

The current classification of garnets, developed by the International Mineralogical Association (IMA) in 2013, proceeds in reverse. It defines the garnet supergroup based on structural type and divides it into five groups and three individual minerals based on chemical aspects, the cation charge on the tetrahedrally coordinated Z-position. In addition, a classification based on the charge distribution across the lattice positions was inserted, and hypothetical end members were added.

Physical properties

Properties

Garnet species are found in every colour, with reddish shades most common. Blue garnets are the rarest and were first reported in the 1990s.

Garnet species' light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral's lustre is categorized as vitreous (glass-like) or resinous (amber-like).

Crystal structure

Garnets are nesosilicates having the general formula X3Y2(SiO4)3. The X site is usually occupied by divalent cations (Ca, Mg, Fe, Mn)2+ and the Y site by trivalent cations (Al3+, Fe3+, Cr3+) in an octahedral/tetrahedral framework with 4− occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit as well as the hexoctahedral habit. They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to one another but are never actually cubic because, despite being isometric, the {100} and {111} families of planes are depleted. Garnets do not have any cleavage planes, so, when they fracture under stress, sharp, irregular (conchoidal) pieces are formed.

Colors

Due to the large number of chemical elements that make them up, garnets have a wide range of colors, ranging from yellow to red, including green and black. Only the color blue is not represented.

Although the predominant idiochromatic color of garnets (i.e. corresponding to the main elements of the mineral) is reddish-brown, due to the presence of iron in pyralspite garnets, ugrandite garnets or calcium garnets are generally not very colorful in themselves, and are therefore particularly sensitive to impurities (allochromatic colorations).

Uvarovite, although it belongs to the ugrandite group, is a striking example of idiochromatic coloration. Its deep green color has the same origin as that of emerald: it is due to the presence of chromium III in an octahedral site in covalent bond with oxygen.

Some secondary chemical elements can substitute for cations in the garnet lattice, and color them allochromatically (relative to impurities). The ions Cr 3+, V 3+ and Ti 3+ or Ti 4+ can give these garnets a whole new appeal and notoriety.

Thus, we can cite the varieties of tsavorite garnet, grossular garnet colored green by the presence of vanadium, and demantoid garnet. The specific green color of the latter is due to the presence of chromium in andradite, itself called melanite when it is colored black by the presence of titanium under the effect of the electronic transition Fe 3+ - Ti 4+ which also colors sapphires blue. Finally, let us not forget the Malaysian garnets, rich in vanadium, which react to UV and then emit colors different from their emission colors under white light.

Some garnets are sometimes star-shaped, and referred to as "stars," when fine, parallel, acicular inclusions create an optical phenomenon called asterism. This process of refracting light in various directions creates the image of a star. These often have four branches, more rarely six.

Hardness

Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.0 to 7.5. The harder species like almandine are often used for abrasive purposes.

Magnetics used in garnet series identification

For gem identification purposes, a pick-up response to a strong neodymium magnet separates garnet from all other natural transparent gemstones commonly used in the jewelry trade. Magnetic susceptibility measurements in conjunction with refractive index can be used to distinguish garnet species and varieties, and determine the composition of garnets in terms of percentages of end-member species within an individual gem.

Garnet group end member species

Pyralspite garnets – aluminium in Y site

Almandine: Fe3Al2(SiO4)3

Pyrope: Mg3Al2(SiO4)3

Spessartine: Mn3Al2(SiO4)3

Almandine

Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the Latin meaning "live coal" or burning charcoal. The name Almandine is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandine is an iron-aluminium garnet with the formula Fe3Al2(SiO4)3; the deep red transparent stones are often called precious garnet and are used as gemstones (being the most common of the gem garnets). Almandine occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle.

Pyrope

Pyrope (from the Greek pyrōpós meaning "firelike") is red in color and chemically an aluminium silicate with the formula Mg3Al2(SiO4)3, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to black. Pyrope and spessartine gemstones have been recovered from the Sloan diamondiferous kimberlites in Colorado, from the Bishop Conglomerate and in a Tertiary age lamprophyre at Cedar Mountain in Wyoming.

A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, Greek for "rose". In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandine, in the proportion of two parts pyrope to one part almandine. Pyrope has tradenames some of which are misnomers; Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian ruby from the Czech Republic.

Pyrope is an indicator mineral for high-pressure rocks. Mantle-derived rocks (peridotites and eclogites) commonly contain a pyrope variety.

Spessartine

Spessartine or spessartite is manganese aluminium garnet, Mn3Al2(SiO4)3. Its name is derived from Spessart in Bavaria. It occurs most often in skarns, granite pegmatite and allied rock types, and in certain low grade metamorphic phyllites. Spessartine of an orange-yellow is found in Madagascar. Violet-red spessartines are found in rhyolites in Colorado and Maine.

Pyrope–spessartine (blue garnet or color-change garnet)

Blue pyrope–spessartine garnets were discovered in the late 1990s in Bekily, Madagascar. This type has also been found in parts of the United States, Russia, Kenya, Tanzania, and Turkey. It changes color from blue-green to purple depending on the color temperature of viewing light, as a result of the relatively high amounts of vanadium (about 1 wt.% V2O3).

Other varieties of color-changing garnets exist. In daylight, their color ranges from shades of green, beige, brown, gray, and blue, but in incandescent light, they appear a reddish or purplish/pink color.

This is the rarest type of garnet. Because of its color-changing quality, this kind of garnet resembles alexandrite.

Ugrandite group – calcium in X site

Andradite: Ca3Fe2(SiO4)3

Grossular: Ca3Al2(SiO4)3

Uvarovite: Ca3Cr2(SiO4)3

Andradite

Andradite is a calcium-iron garnet, Ca3Fe2(SiO4)3, is of variable composition and may be red, yellow, brown, green or black. The recognized varieties are demantoid (green), melanite (black), and topazolite (yellow or green). The red-brown translucent variety of colophonite is recognized as a partially obsolete name. Andradite is found in skarns and in deep-seated igneous rocks like syenite as well as serpentines and greenschists. Demantoid is one of the most prized of garnet varieties.

Grossular

Grossular is a calcium-aluminium garnet with the formula Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Grossular is found in skarns, contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.

Grossular garnet from Kenya and Tanzania has been called tsavorite. Tsavorite was first described in the 1960s in the Tsavo area of Kenya, from which the gem takes its name.

Uvarovite

Uvarovite is a calcium chromium garnet with the formula Ca3Cr2(SiO4)3. This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural Mountains of Russia and Outokumpu, Finland. Uvarovite is named for Count Uvaro, a Russian imperial statesman.

Less common species

Calcium in X site

Goldmanite: Ca3(V3+,Al,Fe3+)2(SiO4)3

Kimzeyite: Ca3(Zr, Ti)2[(Si,Al,Fe3+)O4]3

Morimotoite: Ca3Ti4+Fe2+(SiO4)3

Schorlomite: Ca3Ti4+2(SiO4)(Fe3+O4)2

Hydroxide bearing – calcium in X site

Hydrogrossular: Ca3Al2(SiO4)3−x(OH)4x

Hibschite: Ca3Al2(SiO4)3−x(OH)4x (where x is between 0.2 and 1.5)

Katoite: Ca3Al2(SiO4)3−x(OH)4x (where x is greater than 1.5)

Magnesium or manganese in X site

Knorringite: Mg3Cr2(SiO4)3

Majorite: Mg3(Fe2+Si)(SiO4)3

Calderite: Mn3Fe3+2(SiO4)3

Knorringite

Knorringite is a magnesium-chromium garnet species with the formula Mg3Cr2(SiO4)3. Pure endmember knorringite never occurs in nature. Pyrope rich in the knorringite component is only formed under high pressure and is often found in kimberlites. It is used as an indicator mineral in the search for diamonds.

Garnet structural group

Formula: X3Z2(TO4)3 (X = Ca, Fe, etc., Z = Al, Cr, etc., T = Si, As, V, Fe, Al)

All are cubic or strongly pseudocubic.

IMA/CNMNC Nickel-Strunz Mineral class	Mineral name	Formula	Crystal system	Point group	Space group
04 Oxide	Bitikleite-(SnAl)	Ca3SnSb(AlO4)3	isometric	m3m	Ia3d
04 Oxide	Bitikleite-(SnFe)	Ca3(SnSb5+)(Fe3+O4)3	isometric	m3m	Ia3d
04 Oxide	Bitikleite-(ZrFe)	Ca3SbZr(Fe3+O4)3	isometric	m3m	Ia3d
04 Tellurate	Yafsoanite	Ca3Zn3(Te6+O6)2	isometric	m3m or 432	Ia3d or I4132
08 Arsenate	Berzeliite	NaCa2Mg2(AsO4)3	isometric	m3m	Ia3d
08 Vanadate	Palenzonaite	NaCa2Mn2+2(VO4)3	isometric	m3m	Ia3d
08 Vanadate	Schäferite	NaCa2Mg2(VO4)3	isometric	m3m	Ia3d

IMA/CNMNC – Nickel-Strunz – Mineral subclass: 09.A Nesosilicate

Nickel-Strunz classification: 09.AD.25

Mineral name	Formula	Crystal system	Point group	Space group
Almandine	Fe2+3Al2(SiO4)3	isometric	m3m	Ia3d
Andradite	Ca3Fe3+2(SiO4)3	isometric	m3m	Ia3d
Calderite	Mn+23Fe+32(SiO4)3	isometric	m3m	Ia3d
Goldmanite	Ca3V3+2(SiO4)3	isometric	m3m	Ia3d
Grossular	Ca3Al2(SiO4)3	isometric	m3m	Ia3d
Henritermierite	Ca3Mn3+2(SiO4)2(OH)4	tetragonal	4/mmm	I41/acd
Hibschite	Ca3Al2(SiO4)(3−x)(OH)4x (x= 0.2–1.5)	isometric	m3m	Ia3d
Katoite	Ca3Al2(SiO4)(3−x)(OH)4x (x= 1.5–3)	isometric	m3m	Ia3d
Kerimasite	Ca3Zr2(Fe+3O4)2(SiO4)	isometric	m3m	Ia3d
Kimzeyite	Ca3Zr2(Al+3O4)2(SiO4)	isometric	m3m	Ia3d
Knorringite	Mg3Cr2(SiO4)3	isometric	m3m	Ia3d
Majorite	Mg3(Fe2+Si)(SiO4)3	tetragonal	4/m or 4/mmm	I41/a or I41/acd
Menzerite-(Y)	Y2CaMg2(SiO4)3	isometric	m3m	Ia3d
Momoiite	Mn2+3V3+2(SiO4)3	isometric	m3m	Ia3d
Morimotoite	Ca3(Fe2+Ti4+)(SiO4)3	isometric	m3m	Ia3d
Pyrope	Mg3Al2(SiO4)3	isometric	m3m	Ia3d
Schorlomite	Ca3Ti4+2(Fe3+O4)2(SiO4)	isometric	m3m	Ia3d
Spessartine	Mn2+3Al2(SiO4)3	isometric	m3m	Ia3d
Toturite	Ca3Sn2(Fe3+O4)2(SiO4)	isometric	m3m	Ia3d
Uvarovite	Ca3Cr2(SiO4)3	isometric	m3m	Ia3d

References: Mindat.org; mineral name, chemical formula and space group (American Mineralogist Crystal Structure Database) of the IMA Database of Mineral Properties/ RRUFF Project, Univ. of Arizona, was preferred most of the time. Minor components in formulae have been left out to highlight the dominant chemical endmember that defines each species.

Modifications and varieties

Achtaragdite (also Achtarandite, English Akhtaragdite): Pseudomorph of grossular-katoite solid solutions (hydrogrossular) after a mineral of the Mayenite upper group, possibly also from hibschite after wadalite from Vilyuy in Russia. Achtaragdite is usually found in the form of tetrahedral or triakistetrahedral crystals of white-grey to grey-brown color.

Bredbergite (after James Dwight Dana, around 1900): Obsolete and no longer used name for a magnesium-rich andradite variety

Demantoid (after Nils von Nordenskiöld, around 1870): Andradite variety colored yellow-green by foreign impurities

Melanite (after Abraham Gottlob Werner, 1799): Is considered a titanium-rich variety of andradite and was named after the Greek word μέλας for black, as it occurs predominantly in grey-black to pitch-black crystals or coarse aggregates.

Topazolite (after PC Bonvoisin, 1806): Light yellow, “ topaz-like ” andradite variety, first discovered in the Valle di Lanza in the Italian region of Piedmont 

Xalostocite: Name for a dense intergrowth of translucent pink grossulars with white marble, named after the locality of Xalostoc in the Mexican state of Morelos. 

Synthetic garnets

The crystallographic structure of garnets has been expanded from the prototype to include chemicals with the general formula A3B2(CO4)3. Besides silicon, a large number of elements have been put on the C site, including germanium, gallium, aluminum, vanadium and iron.

Yttrium aluminium garnet (YAG), Y3Al2(AlO4)3, is used for synthetic gemstones. Due to its fairly high refractive index, YAG was used as a diamond simulant in the 1970s until the methods of producing the more advanced simulant cubic zirconia in commercial quantities were developed. When doped with neodymium (Nd3+), erbium or gadolinium YAG may be used as the lasing medium in Nd:YAG lasers, Er:YAG lasers and Gd:YAG lasers respectively. These doped YAG lasers are used in medical procedures including laser skin resurfacing, dentistry, and ophthalmology.

Interesting magnetic properties arise when the appropriate elements are used. In yttrium iron garnet (YIG), Y3Fe2(FeO4)3, the five iron(III) ions occupy two octahedral and three tetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit different spins, resulting in magnetic behavior. YIG is a ferrimagnetic material having a Curie temperature of 550 K. Yttrium iron garnet can be made into YIG spheres, which serve as magnetically tunable filters and resonators for microwave frequencies.

Lutetium aluminium garnet (LuAG), Al5Lu3O12, is an inorganic compound with a unique crystal structure primarily known for its use in high-efficiency laser devices. LuAG is also useful in the synthesis of transparent ceramics. LuAG is particularly favored over other crystals for its high density and thermal conductivity; it has a relatively small lattice constant in comparison to the other rare-earth garnets, which results in a higher density producing a crystal field with narrower linewidths and greater energy level splitting in absorption and emission.

Terbium gallium garnet (TGG), Tb3Ga5O12, is a Faraday rotator material with excellent transparency properties and is very resistant to laser damage. TGG can be used in optical isolators for laser systems, in optical circulators for fiber optic systems, in optical modulators, and in current and magnetic field sensors.

Another example is gadolinium gallium garnet (GGG), Gd3Ga2(GaO4)3 which is synthesized for use as a substrate for liquid-phase epitaxy of magnetic garnet films for bubble memory and magneto-optical applications.

Geological importance

The mineral garnet is commonly found in metamorphic and to a lesser extent, igneous rocks. Most natural garnets are compositionally zoned and contain inclusions. Its crystal lattice structure is stable at high pressures and temperatures and is thus found in green-schist facies metamorphic rocks including gneiss, hornblende schist, and mica schist. The composition that is stable at the pressure and temperature conditions of Earth's mantle is pyrope, which is often found in peridotites and kimberlites, as well as the serpentines that form from them. Garnets are unique in that they can record the pressures and temperatures of peak metamorphism and are used as geobarometers and geothermometers in the study of geothermobarometry which determines "P-T Paths", Pressure-Temperature Paths. Garnets are used as an index mineral in the delineation of isograds in metamorphic rocks. Compositional zoning and inclusions can mark the change from growth of the crystals at low temperatures to higher temperatures. Garnets that are not compositionally zoned more than likely experienced ultra high temperatures (above 700 °C) that led to diffusion of major elements within the crystal lattice, effectively homogenizing the crystal or they were never zoned. Garnets can also form metamorphic textures that can help interpret structural histories.

In addition to being used to devolve conditions of metamorphism, garnets can be used to date certain geologic events. Garnet has been developed as a U-Pb geochronometer, to date the age of crystallization as well as a thermochronometer in the (U-Th)/He system to date timing of cooling below a closure temperature.

Garnets can be chemically altered and most often alter to serpentine, talc, and chlorite.

Largest garnet crystal

The open-pit Barton Garnet Mine, located at Gore Mountain in the Adirondack Mountains, yields the world's largest single crystals of garnet; diameters range from 5 to 35 cm and commonly average 10–18 cm.

Gore Mountain garnets are unique in many respects, and considerable effort has been made to determine the timing of garnet growth. The first dating was that of Basu et al. (1989), who used plagioclase-hornblende-garnet to produce a Sm/Nd isochron that yielded an age of 1059 ± 19 Ma. Mezger et al. (1992) conducted their own Sm/Nd investigation using hornblende and the drilled core of a 50 cm garnet to produce an isochron age of 1051 ± 4 Ma. Connelly (2006) utilized seven different fractions of a Gore Mountain garnet to obtain a Lu-Hf isochron age of 1046.6 ± 6 Ma. It is therefore concluded with confidence that the garnets formed at 1049 ± 5 Ma, the average of the three determinations. This is also the local age of peak metamorphism in the 1090–1040 Ma Ottawan phase of the Grenvillian orogeny and serves as a critical data point in ascertaining the evolution of the megacrystic garnet deposits.

Uses

Gemstones

Garnets are used in various forms as gemstones . Among them are dark-red pyrope, also known as cape ruby; reddish-black almandine; emerald-green uvarovite; yellow-green andradite; black schorlomite and melanite; transparent-green demantoid; and orange-red spessartine. There is also grossular. A new variety, the orange mandarin garnet, has also been around for a few years. Garnets are also known as the common man's gems.

Red garnets were the most commonly used gemstones in the Late Antique Roman world, and the Migration Period art of the "barbarian" peoples who took over the territory of the Western Roman Empire. They were especially used inlaid in gold cells in the cloisonné technique, a style often just called garnet cloisonné, found from Anglo-Saxon England, as at Sutton Hoo, to the Black Sea. Thousands of Tamraparniyan gold, silver and red garnet shipments were made in the old world, including to Rome, Greece, the Middle East, Serica and Anglo Saxons; recent findings such as the Staffordshire Hoard and the pendant of the Winfarthing Woman skeleton of Norfolk confirm an established gem trade route with South India and Tamraparni (ancient Sri Lanka), known from antiquity for its production of gemstones.

Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow, and orange. IThe garnet family is one of the most complex in the gem world. It is not a single species, but is composed of multiple species and varieties.

Almandine garnet is the state mineral of Connecticut, star garnet is the state gemstone of Idaho, garnet is the state gemstone of New York, and grossular garnet is the state gemstone of Vermont.

Industrial uses

Garnet sand is a good abrasive, and a common replacement for silica sand in abrasive blasting operations. Alluvial garnet grains which are rounder are more suitable for such blasting treatments. Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets. For water jet cutting, garnet extracted from hard rock is suitable since it is more angular in form, therefore more efficient in cutting.

Garnet paper is favored by cabinetmakers for finishing bare wood.

Garnet sand is also used for water filtration media.

As an abrasive, garnet can be broadly divided into two categories; blasting grade and water jet grade. The garnet, as it is mined and collected, is crushed to finer grains; all pieces which are larger than 60 mesh (250 micrometers) are normally used for sand blasting. The pieces between 60 mesh (250 micrometers) and 200 mesh (74 micrometers) are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh (74 micrometers) are used for glass polishing and lapping. Regardless of the application, the larger grain sizes are used for faster work and the smaller ones are used for finer finishes.

There are different kinds of abrasive garnets which can be divided based on their origin. The largest source of abrasive garnet today is garnet-rich beach sand which is quite abundant on Indian and Australian coasts and the main producers today are Australia and India.

This material is particularly popular due to its consistent supplies, huge quantities and clean material. The common problems with this material are the presence of ilmenite and chloride compounds. Since the material has been naturally crushed and ground on the beaches for past centuries, the material is normally available in fine sizes only. Most of the garnet at the Tuticorin beach in south India is 80 mesh, and ranges from 56 mesh to 100 mesh size.

River garnet is particularly abundant in Australia. The river sand garnet occurs as a placer deposit.

Rock garnet is perhaps the garnet type used for the longest period of time. This type of garnet is produced in America, China and western India. These crystals are crushed in mills and then purified by wind blowing, magnetic separation, sieving and, if required, washing. Being freshly crushed, this garnet has the sharpest edges and therefore performs far better than other kinds of garnet. Both the river and the beach garnet suffer from the tumbling effect of hundreds of thousands of years which rounds off the edges. Gore Mountain Garnet from Warren County, New York, USA, is a significant source of rock garnet for use as an industrial abrasive.

Traditions and beliefs

Various beliefs attribute great virtues to garnets:

According to Vedic astrology, hessonite garnet is a talisman that protects against demonic influences from the celestial body named Rahu.

Garnet is also considered a sacred stone by many Native American peoples.

According to popular belief, garnet is believed to protect against injury and poison, stop bleeding, symbolize truth and fidelity, and bring prosperity.

The semi-precious stone traditionally associated by jewelers with the month of January with regard to births.

Sourced from Wikipedia

Howlite

Howlite, a calcium borosilicate hydroxide (Ca2B5SiO9(OH)5), is a borate mineral found in evaporite deposits.

Howlite

General

Category Inoborates

Formula Ca2B5SiO9(OH)5

IMA symbol How

Strunz classification 6.CB.20

Dana classification 25.3.5.1

Crystal system Monoclinic

Crystal class Prismatic (2/m)

(same H-M symbol)

Space group P21/c

Unit cell a = 12.82 Å, b = 9.351(1) Å

c = 8.608(2) Å; β = 104.84(2)°; Z = 4

Identification

Color White, colorless

Crystal habit Massive to nodular, occurs as tabular prisms flattened parallel to 

Cleavage None

Fracture Conchoidal, uneven

Mohs scale hardness 3.5

Luster Subvitreous, glimmering

Streak white

Diaphaneity Translucent

Specific gravity 2.53 – 2.59

Optical properties Biaxial (−), colorless (transmitted light)

Refractive index nα = 1.583 – 1.586 nβ = 1.596 – 1.598 nγ = 1.600

Birefringence δ = 0.017

2V angle 73°

History

Howlite was discovered near Windsor, Nova Scotia, in 1868 by Henry How (1828–1879), a Canadian chemist, geologist, and mineralogist. How was alerted to the unknown mineral by miners in a gypsum quarry, who found it to be a nuisance. He called the new mineral silico-boro-calcite; it was given the name howlite by the American geologist James Dwight Dana shortly thereafter.

Classification

In the outdated 8th edition of the mineral classification according to Strunz, howlite belonged to the mineral class of “silicates” and there to the department “neso-subsilicates”, where it is together with dumortierite, garrelsite, grandidierite, harkerite, kornerupine, melanocerite, painite and serendibite in the “dumortierite-grandierite group” with the system number VIII/A'.13.

In the Lapis classification system by Stefan Weiß, last revised in 2018 and formally based on the 8th edition of Karl Hugo Strunz 's old system, the mineral was given the system and mineral number VIII/F.28-010. This corresponds to the class of "silicates" and the division "chain and band silicates," where howlite is the only mineral forming an unnamed group with the system number VIII/F.28. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), classifies howlite in the class of "borates" and within it in the division "triborates." Here, the mineral is found in the subdivision "chain and ribbon triborates (inotriborates)," where it is the sole member of an unnamed group with the system number 6.CB.20.

In the Dana classification of minerals, which is primarily used in English-speaking countries, howlite has the system and mineral number 25.03.05.01. This corresponds to the class "Carbonates, Nitrates, and Borates" and the division "Anhydrous Borates with Hydroxyl or Halogen." Here, it is found within the subdivision "Anhydrous Borates with Hydroxyl or Halogen" as the sole member of an unnamed group with the system number 25.03.05.

Crystal structure

Howlite crystallizes monoclinic in the space group P 2 1 / c (space group no. 14) with the lattice parameters a = 12.82 Å; b = 9.35 Å; c = 8.61 Å and β = 104.8° as well as 4 formula units per unit cell. 

Characteristics

Howlite is often interwoven with brown-black veins, whose marbled pattern resembles the coveted matrix of turquoise. The crystal faces exhibit a vitreous luster, while the fracture surfaces are matte. Howlite fractures conch-like, similar to glass.

Howlite is very similar in appearance to magnesite (a magnesium carbonate) and can often only be distinguished by chemical analysis. Howlite melts into a gel when exposed to hydrochloric acid, whereas magnesite reacts by releasing gas. A density determination is often insufficient to distinguish between the two, as howlite, with a density of 2.5 to 2.6 g/cm³, is very similar to magnesite (2.9 to 3.1 g/cm³). Furthermore, magnesite is often even lighter due to air inclusions. 

Chemical characteristics

It was formerly classified as a calciumsilicatecontainingboronanions, but the study of its structure indicates that it is acalciumborate that also containssilicon. In addition to the elements in its formula, it usually contains impurities such assodiumandpotassium.

Geology

The most common form of howlite is irregular nodules, sometimes resembling cauliflower. Crystals of howlite are rare, having been found in only a couple localities worldwide. Crystals were first reported from Tick Canyon in the Sierra Pelona Mountains of California, and later at Iona, Nova Scotia. Crystals reach a maximum size of about one centimeter. The nodules are white with fine grey or black veins in an erratic, often web-like pattern, opaque with a sub-vitreous luster. The crystals at Iona are colorless, white or brown and are often translucent or transparent.

Its structure is monoclinic with a Mohs hardness of 3.5 and lacks regular cleavage. Crystals are prismatic and flattened on {100}. The crystals from Tick Canyon are elongated along the 010 axis, while those from Iona are elongated along the 001 axis.

Jewelry

Howlite is commonly used to make decorative objects such as small carvings or jewelry components. Because of its porous texture, howlite can be easily dyed to imitate other minerals, especially turquoise because of the superficial similarity of the veining patterns. Due to its low Mohs hardness of 3 to 3.5, it is very soft and must therefore be protected from damage by stabilization (coating with resin or plastic) to protect it from daily use. Howlite is also sold in its natural state, sometimes under the trade names of "white turquoise" or "white buffalo turquoise," or the derived name "white buffalo stone" and is used to produce jewelry similar to how turquoise is used. 

Manipulations and imitations

Because magnesite looks so similar to howlite, but is more common and therefore cheaper, it is often mistakenly sold as howlite. Both, however, are dyed blue and serve as imitations of the rare and valuable matrix turquoise.

Esotericism

In esotericism, howlite is said to have a special psychological and physical effect, for which, however, there is no scientific evidence so far.

Among other things, it is said to strengthen judgment and memory, reduce stress, feelings of pain, and anger, neutralize negative energies, and release blockages. Howlite water is said to have a generally positive effect on problems with bones, teeth, joints, and nails.

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Jade

Jade is an umbrella term for two different types of decorative rocks used for jewelry or ornaments. Jade is often referred to by either of two different silicate mineral names: nephrite (a silicate of calcium and magnesium in the amphibole group of minerals), or jadeite (a silicate of sodium and aluminum in the pyroxene group of minerals). Nephrite is typically green, although may be yellow, white or black. Jadeite varies from white or near-colorless, through various shades of green (including an emerald green, termed 'imperial'), to lavender, yellow, orange, brown and black. Rarely it may be blue. Both of these names refer to their use as gemstones, and each has a mineralogically more specific name. Both the amphibole jade (nephrite) and pyroxene jade are mineral aggregates (rocks) rather than mineral species.

Nephrite was deprecated by the International Mineralogical Association as a mineral species name in 1978 (replaced by tremolite). The name "nephrite" is mineralogically correct for referring to the rock. Jadeite is a legitimate mineral species, differing from the pyroxene jade rock. In China, the name jadeite also known as fei cui, the traditional Chinese name for this gem that was in use long before Damour created the name in 1863.

Jade is well known for its ornamental use in East Asian, South Asian, and Southeast Asian art. It is commonly used in Latin America, such as Mexico and Guatemala. The use of jade in Mesoamerica for symbolic and ideological ritual was influenced by its rarity and value among pre-Columbian Mesoamerican cultures, such as the Olmecs, the Maya, and other ancient civilizations of the Valley of Mexico.

Jade is classified into three main types: Type A, Type B, and Type C. Type A jade refers to natural, untreated jadeite jade, prized for its purity and vibrant colors. It is the most valuable and sought-after type, often characterized by its vivid green hues and high translucency. Type A jade is revered for its symbolism of purity, harmony, and protection in various cultures, especially in East Asia where it holds significant cultural and spiritual importance. Types B and C have been enhanced with resin and colourant respectively.

Jade

General

Category Minerals

Crystal system Monoclinic

Identification

Color Virtually all colors, mostly green

Crystal habit Intergrown grainy or fine fibrous aggregate

Cleavage None

Fracture Splintery

Tenacity Brittle

Mohs scale hardness 6–7

Diaphaneity Translucent, opaque

Specific gravity 2.9–3.38

Refractive index 1.600–1.688

Birefringence 0.020–0.027

Pleochroism Absent

Dispersion None

Etymology

The English word jade is derived (via French l'ejade and Latin ilia 'flanks, kidney area') from the Spanish term piedra de ijada (first recorded in 1565) or 'loin stone', from its reputed efficacy in curing ailments of the loins and kidneys. Nephrite is derived from lapis nephriticus, a Latin translation of the Spanish piedra de ijada.

Classification

Jade is not actually a mineral, but rather a stone made of particular minerals (jadeite and nephrite) distributed within it in a structure made of very fine grains and interwoven fibers. Jadeite is formed only in small quantities as crystallines.

From the beginning, other minerals were sold under the name of jade; these were later more appropriately called "serpentine". Serpentine not only has the same appearance as jade, but also appears in the same deposits as jadeite and nephrite; however, it is more workable and less resistant than true jade.

Jade is a very hard mineral that is worked by grinding with abrasives. It is subject to discoloration and decomposition and has the property of heating up rapidly and being sonorous. The wide range of colors is due to small and different quantities of different metal oxides such as iron, chromium and manganese. The colors can vary from green to milky white, from brown to dark gray, from yellow to black. Siberian nephrite is dark green dotted with small black spots, while jadeite is of a purer and more vivid color due to the presence of chromium.

Types

Jade actually refers to three distinct minerals that can have quite similar appearances and properties: jadeite, nephrite, and kosmochlor(sodium and chromium silicate).

Initially, the first two minerals were not differentiated. It was in 1863 that Alexis Damour described and named jadeite, after observing a variety of jade whose composition differed radically from that of another variety he had studied in 1846 (nephrite) and identified as belonging to the tremolite family. As a result of his work, the following distinction was established:

nephrite jade, composed mainly of nephrite, a calcium and magnesium silicate of the amphibole group, quite common;

jadeite jade, composed mainly of jadeite, a sodium and aluminum silicate from the pyroxene group, harder, denser, rarer and considered more precious;

As for the third mineral in the category, kosmochlor, its physicochemical characteristics (sodium and chromium silicate) allow it to be associated with jadeite.

Color

Jade is generally a more or less pronounced green.

White jade is pure jade.

Green jade contains chromium salts.

Blue-green jade contains cobalt salts.

Black jade contains titanium salts.

Pink jade contains iron and manganese salts.

History

Jade is considered one of the most remarkable minerals in existence, both as a gemstone and because of its cultural and historical significance, including the belief in its healing properties. But it is not even a mineral in the strict sense. Jade is too valuable and too difficult to work to be used as a raw material for tools. Even the production of a simple amulet can take days. Therefore, only ceremonial weapons and tools were made from it, such as the jade axes found in Central Europe, France, Switzerland and England. Apart from that, jade was and still is often made into jewelry, used in arts and crafts and, due to the magical qualities sometimes attributed to the material - in the broadest sense - it is used in corresponding cultic or magical rituals. Jade was either of independent artistic significance and jade processing developed its own styles, or where a spiritual-religious world of ideas arose that had jade as its subject. 

East Asia

Prehistoric and historic China

During Neolithic times, the key known sources of nephrite jade in China for utilitarian and ceremonial jade items were the now-depleted deposits in the Ningshao area in the Yangtze River Delta (Liangzhu culture 3400–2250 BC) and in an area of the Liaoning province and Inner Mongolia (Hongshan culture 4700–2200 BC). Dushan Jade (a rock composed largely of anorthite feldspar and zoisite) was being mined as early as 6000 BC. In the Yin Ruins of the Shang Dynasty (1600 to 1050 BC) in Anyang, Dushan Jade ornaments were unearthed in the tomb of the Shang kings.

Jade was considered to be the "imperial gem" and was used to create many utilitarian and ceremonial objects, from indoor decorative items to jade burial suits. From the earliest Chinese dynasties to the present, the jade deposits most used were not only those of Khotan in the Western Chinese province of Xinjiang but other parts of China as well, such as Lantian, Shaanxi. There, white and greenish nephrite jade is found in small quarries and as pebbles and boulders in the rivers flowing from the Kuen-Lun mountain range eastward into the Takla-Makan desert area. The river jade collection is concentrated in the Yarkand, the White Jade (Yurungkash) and Black Jade (Karakash) Rivers. From the Kingdom of Khotan, on the southern leg of the Silk Road, yearly tribute payments consisting of the most precious white jade were made to the Chinese Imperial court and there worked into objets d'art by skilled artisans as jade had a status-value exceeding that of gold or silver. Jade became a favourite material for the crafting of Chinese scholars' objects, such as rests for calligraphy brushes.

Jadeite, with its bright emerald-green, lavender, pink, orange, yellow, red, black, white, near-colorless and brown colors was imported from Burma to China in quantity only after about 1800. The vivid white to green variety became known as fei cui (翡翠) or kingfisher jade, due to its resemblance to the feathers of the kingfisher bird. That definition was later expanded to include all other colors that the rock is found in. It quickly became almost as popular as nephrite and a favorite of aristocracy, while scholars still had strong attachment to nephrite (white jade, or Hetian jade), which they deemed to be the symbol of a nobleman.

In the history of the art of the Chinese empire, jade has had a special significance, comparable with that of gold and diamonds in the West. Jade was used for the finest objects and cult figures, and for grave furnishings for high-ranking members of the imperial family. Due to that significance and the rising middle class in China, in 2010 the finest jade when found in nuggets of "mutton fat" jade – so-named for its marbled white consistency – could sell for $3,000 an ounce, a tenfold increase from a decade previously.

The Chinese character 玉 (yù) is used to denote the several types of stone known in English as "jade" (e.g. 玉器, jadewares), such as jadeite (硬玉, 'hard jade', another name for 翡翠) and nephrite (軟玉, 'soft jade'). While still in use, the terms "hard jade" and "soft jade" resulted from a mistranslation by a Japanese geologist, and should be avoided.

But because of the value added culturally to jades throughout Chinese history, the word has also come to refer more generally to precious or ornamental stones, and is very common in more symbolic usage as in phrases like 拋磚引玉/抛砖引玉 (lit. "casting a brick (i.e. the speaker's own words) to draw a jade (i.e. pearls of wisdom from the other party)"), 玉容 (a beautiful face; "jade countenance"), and 玉立 (slim and graceful; "jade standing upright"). The character has a similar range of meanings when appearing as a radical as parts of other characters.

Prehistoric and historic Japan

Jade in Japan was used for jade bracelets. It was a symbol of wealth and power. Leaders also used jade in rituals. It is the national stone of Japan. Examples of use in Japan can be traced back to the early Jomon period about 7,000 years ago. XRF analysis results have revealed that all jade used in Japan since the Jomon period is from Itoigawa. The jade culture that blossomed in ancient Japan respected green ones, and jade of other colors was not used. There is a theory that the reason why the meaning is that it was believed that the color of green enables the reproduction of fertility, the life, and the soul of the earth.

Prehistoric and historic Korea

The use of jade and other greenstone was a long-term tradition in Korea (c. 850 BC – AD 668). Jade is found in small numbers of pit-houses and burials. The craft production of small comma-shaped and tubular "jades" using materials such as jade, microcline, jasper, etc., in southern Korea originates from the Middle Mumun Pottery Period (c. 850–550 BC). Comma-shaped jades are found on some of the gold crowns of Silla royalty (c. 300/400–668 AD) and sumptuous elite burials of the Korean Three Kingdoms. After the state of Silla united the Korean Peninsula in 668, the widespread popularisation of death rituals related to Buddhism resulted in the decline of the use of jade in burials as prestige mortuary goods.

South Asia

India

The Jain temple of Kolanpak in the Nalgonda district, Telangana, India is home to a 5-foot (1.5 m) high sculpture of Mahavira that is carved entirely out of jade. India is also noted for its craftsman tradition of using large amounts of green serpentine or false jade obtained primarily from Afghanistan in order to fashion jewellery and ornamental items such as sword hilts and dagger handles.

The Salar Jung Museum in Hyderabad has a wide range of jade hilted daggers, mostly owned by the former Sultans of Hyderabad.

Southeast Asia

Myanmar

Today, it is estimated that Myanmar is the origin of upwards of 70% of the world's supply of high-quality jadeite. Most of the jadeite mined in Myanmar is not cut for use in Myanmar, instead being transported to other nations, primarily in Asia, for use in jewelry and other products. The jadeite deposits found in Kachinland, in Myanmar's northern regions is the highest quality jadeite in the world, considered precious by sources in China going as far back as the 10th century.

Jadeite in Myanmar is primarily found in the "Jade Tract" located in Lonkin Township in Kachin State in northern Myanmar which encompasses the alluvial region of the Uyu River between the 25th and 26th parallels. Present-day extraction of jade in this region occurs at the Phakant-gyi, Maw Sisa, Tin Tin, and Khansee mines. Khansee is also the only mine that produces maw sit sit, a kosmochlor-rich jade rock. Mines at Tawmaw and Hweka are mostly exhausted. From 1964 to 1981, mining was exclusively an enterprise of the Myanmar government. In 1981, 1985, and 1995, the Gemstone laws were modified to allow increasing private enterprise. In addition to this region, there are also notable mines in the neighboring Sagaing District, near the towns of Nasibon and Natmaw and Hkamti. Sagaing is a district in Myanmar proper, not a part of the ethic Kachin State.

Southeast Asia

Carved nephrite jade was the main commodity trade of an extensive prehistoric trading network connecting multiple areas in Southeast Asia. The nephrite jade was mined in eastern Taiwan by the animist Taiwanese indigenous peoples and processed mostly in the Philippines by the animist indigenous Filipinos. Some were also processed in Vietnam, while the peoples of Brunei, Cambodia, Indonesia, Malaysia, Singapore, and Thailand also participated in the massive animist-led nephrite jade trading network, where other commodities were also traded. Participants in the network at the time had a majority animist population. The maritime road is one of the most extensive sea-based trade networks of a single geological material in the prehistoric world. It was in existence for at least 3,000 years, where its peak production was from 2000 BCE to 500 CE, older than the Silk Road in mainland Eurasia. It began to wane during its final centuries from 500 CE until 1000 CE. The entire period of the network was a golden age for the diverse animist societies of the region.

Others

Māori

Nephrite jade in New Zealand is known as pounamu in the Māori language (often called "greenstone" in New Zealand English), and plays an important role in Māori culture. It is considered a taonga, or treasure, and therefore protected under the Treaty of Waitangi, and the exploitation of it is restricted and closely monitored. It is found only in the South Island of New Zealand, known as Te Wai Pounamu in Māori—"The Greenstone Water", or Te Wahi Pounamu—"The Place of Greenstone".

Pounamu taonga increase in mana (prestige) as they pass from one generation to another. The most prized taonga are those with known histories going back many generations. These are believed to have their own mana and were often given as gifts to seal important agreements.

Tools, weapons and ornaments were made of it; in particular adzes, the 'mere' (short club), and the hei-tiki (neck pendant). Nephrite jewellery of Maori design is widely popular with locals and tourists, although some of the jade used for these is now imported from British Columbia and elsewhere.

Pounamu taonga include tools such as toki (adzes), whao (chisels), whao whakakōka (gouges), ripi pounamu (knives), scrapers, awls, hammer stones, and drill points. Hunting tools include matau (fishing hooks) and lures, spear points, and kākā poria (leg rings for fastening captive birds); weapons such as mere (short handled clubs); and ornaments such as pendants (hei-tiki, hei matau and pekapeka), ear pendants (kuru and kapeu), and cloak pins. Functional pounamu tools were widely worn for both practical and ornamental reasons, and continued to be worn as purely ornamental pendants (hei kakï) even after they were no longer used as tools.

Mesoamerica

Jade was a rare and valued material in pre-Columbian Mesoamerica. The only source from which the various indigenous cultures, such as the Olmec and Maya, could obtain jade was located in the Motagua River valley in Guatemala. Jade was largely an elite good, and was usually carved in various ways, whether serving as a medium upon which hieroglyphs were inscribed, or shaped into symbolic figurines. Generally, the material was highly symbolic, and it was often employed in the performance of ideological practices and rituals.

Canada

Jade was first identified in Canada by Chinese settlers in 1886 in British Columbia. At this time jade was considered worthless because the settlers were searching for gold. Jade was not commercialized in Canada until the 1970s. The mining business Loex James Ltd., which was started by two Californians, began commercial mining of Canadian jade in 1972.

Mining is done from large boulders that contain bountiful deposits of jade. Jade is exposed using diamond-tipped core drills in order to extract samples. This is done to ensure that the jade meets requirements. Hydraulic spreaders are then inserted into cleavage points in the rock so that the jade can be broken away. Once the boulders are removed and the jade is accessible, it is broken down into more manageable 10-tonne pieces using water-cooled diamond saws. The jade is then loaded onto trucks and transported to the proper storage facilities.

Russia

Russia imported jade from China for a long time, but in the 1860s its own jade deposits were found in Siberia. Today, the main deposits of jade are located in Eastern Siberia, but jade is also extracted in the Polar Urals and in the Krasnoyarsk territory (Kantegirskoye and Kurtushibinskoye deposits). Russian raw jade reserves are estimated at 336 tons. Russian jade culture is closely connected with such jewellery production as Fabergé, whose workshops combined the green stone with gold, diamonds, emeralds, and rubies.

Siberia

In the 1950s and 1960s, there was a strong belief among many Siberians, which stemmed from tradition, that jade was part of a class of sacred objects that had life.

Mongolia

In the 1950s and 1960s, there was a strong belief among many Mongolians, which came from ancient tradition, that jade was part of a class of sacred objects that had life.

The mineral

Nephrite and jadeite

It was not until 1863 that French mineralogist Alexis Damour determined that what was referred to as "jade" could in fact be one of two different minerals, either nephrite or jadeite.

Nephrite consists of a microcrystalline interlocking fibrous matrix of the calcium, magnesium-iron rich amphibole mineral series tremolite (calcium-magnesium)-ferroactinolite (calcium-magnesium-iron). The middle member of this series with an intermediate composition is called actinolite (the silky fibrous mineral form is one form of asbestos). The higher the iron content, the greener the colour. Tremolite occurs in metamorphosed dolomitic limestones, and actinolite in metamorphic greenschists/glaucophane schists.

Jadeite is a sodium- and aluminium-rich pyroxene. The more precious kind of jade, this is a microcrystalline interlocking growth of crystals (not a fibrous matrix as nephrite is.) It only occurs in metamorphic rocks.

Both nephrite and jadeite were used from prehistoric periods for hardstone carving. Jadeite has about the same hardness (between 6.0 and 7.0 Mohs hardness) as quartz, while nephrite is slightly softer (6.0 to 6.5) and so can be worked with quartz or garnet sand, and polished with bamboo or even ground jade. However nephrite is tougher and more resistant to breakage. Among the earliest known jade artifacts excavated from prehistoric sites are simple ornaments with bead, button, and tubular shapes. Additionally, jade was used for adze heads, knives, and other weapons, which can be delicately shaped.

As metal-working technologies became available, the beauty of jade made it valuable for ornaments and decorative objects.

Unusual varieties

The name Nephrite derives from the Greek word meaning "kidney". This is because in ancient times it was believed that wearing this kind of jade around the waist could cure kidney disease.

Nephrite can be found in a creamy white form (known in China as "mutton fat" jade) as well as in a variety of light green colours, whereas jadeite shows more colour variations, including blue, brown, red, black, dark green, lavender and white. Of the two, jadeite is rarer, documented in fewer than 12 places worldwide. Translucent emerald-green jadeite is the most prized variety, both historically and today. As "quetzal" jade, bright green jadeite from Guatemala was treasured by Mesoamerican cultures, and as "kingfisher" jade, vivid green rocks from Burma became the preferred stone of post-1800 Chinese imperial scholars and rulers. Burma (Myanmar) and Guatemala are the principal sources of modern gem jadeite. In the area of Mogaung in the Myitkyina District of Upper Burma, jadeite formed a layer in the dark-green serpentine, and has been quarried and exported for well over a hundred years. Canada provides the major share of modern lapidary nephrite.

Enhancement

Jade may be enhanced (sometimes called "stabilized"). Some merchants will refer to these as grades, but degree of enhancement is different from colour and texture quality. In other words, Type A jadeite is not enhanced but can have poor colour and texture. There are three main methods of enhancement, sometimes referred to as the ABC Treatment System:

Type A jadeite has not been treated in any way except surface waxing.

Type B treatment involves exposing a promising but stained piece of jadeite to chemical bleaches and/or acids and impregnating it with a clear polymer resin. This results in a significant improvement of transparency and colour of the material. Currently, infrared spectroscopy is the most accurate test for the detection of polymer in jadeite.

Type C jade has been artificially stained or dyed. The effects are somewhat uncontrollable and may result in a dull brown. In any case, translucency is usually lost.

B+C jade is a combination of B and C: it has been both impregnated and artificially stained.

Type D jade refers to a composite stone such as a doublet comprising a jade top with a plastic backing.

Use as a gemstone

The most valuable jade variety is the so-called Imperial Jade. It is extremely expensive and costs between $5,000 and $8,000 per carat in Hong Kong. One carat is 200 milligrams, and one gram costs up to $40,000, roughly the same as a flawless, intensely blue 1-carat diamond (€36,000). For comparison: 1 g of gold costs approximately $37.50 (as of February 1, 2013). Imperial jade is typically colored with a slightly transparent emerald green. 

Processing and maintenance

Although apparently only slightly above medium hardness (hardness scale 6.5-7), jade is difficult to work due to its delicate consistency, but especially due to its great toughness (tenacity), primarily because its cleavage is not immediately apparent and it breaks in a conch-like manner. It is therefore said of jade artisans in China that they first felt a piece of jade for years and explored its consistency before they began carving and grinding (with sand). Jade cannot be simply carved with a knife like wood, but must be shaped into the desired form in time-consuming steps using disk saws, contour drilling and grinding using simple tools that, apart from the material, have essentially remained unchanged to this day. In the past, and even in the Neolithic period, solid drill heads made of stone or hardwood, but also tubular drill bits made of bone and the very hard bamboo, which in East Asia east of the Movius Line even replaced stone tools. They were rotated using a bowstring wrapped around them. Quartz sand (hardness 7) was often used as an abrasive, which was mixed with water and grease into the drill joint. The harder the abrasive, the more precise the drilling. 

Jadeite is sensitive to heat. This stone is relatively insensitive to acids, but becomes highly sensitive if it has previously been exposed to heat. This means that all acids, acid mixtures (e.g., decoctions), electroplating baths, etc., must be strictly avoided. Exposed jadeite must be protected from spotlights or strong sunlight. It must not be cleaned with ultrasound. Some silver immersion baths leave stains on the stone's surface. For silver settings with jadeite, silver cleaning cloths are recommended. 

Industry

Myanmar

The jade trade in Myanmar consists of the mining, distribution, and manufacture of jadeite—a variety of jade—in the nation of Myanmar (Burma). The jadeite deposits found in Myanmar's northern regions are the source of the highest quality jadeite in the world, and produces upward of 70 percent of the world's supply of high-quality jadeite. Most of the Myanmar's jadeite is exported to other nations, primarily Asian, for use in jewellery, art, and ornaments.

Imitations and manipulations

Since time immemorial, attempts have been made to classify new minerals and rocks under the term jade, primarily for economic reasons. The most well-known and interesting case of this, albeit unintentional, imitation is probably noble serpentine. Serpentine not only looks similar to jade, it even occurs in the same deposits as jadeite and nephrite. However, the material is significantly softer (hardness 4) and has a much lower toughness than jade. Since serpentine is much easier to work, it has become the preferred substitute for jade in recent years. This "noble serpentine" is also mined and processed in Austria, for example (in the town of Bernstein in Burgenland). 

Other jade imitations include:

Prasem or "African Jade" 

various chlorites under the trade names "Marble Bar Jade" and "Pilbara Jade" 

Since 1998, hydrofluorite has been marketed under the trade name Lavender Jade as a smithsonite and jade imitation. 

the green jet stone (emeraldite jade)

green grossular from South Africa (Transvaal jade)

brown vesuvian from California (vesuvian jade, californite)

Serpentine from China (serpentine jade)

Ophicalcite, a type of serpentine marble with a breccia structure, originating from Greece, is often sold under the name "Connemara" or "Verde antique". 

greenish sillimanite from Burma and Sri Lanka (sillimanite jade).

Yellow aragonite, among other materials, is used as a substitute for the rare yellow jadeite. Colored chalcedony and artificial products made of colored glass (trade name "Siberian jade") are also used as imitations. They all differ from genuine jadeite in hardness, specific gravity, and light refraction (and, above all, in price). 

Sourced from Wikipedia