Turquoise

Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8•4H2O. It is rare and valuable in finer grades and has been prized as a gemstone for millennia due to its hue.

Like most other opaque gems, turquoise has been devalued by the introduction of treatments, imitations, and synthetics into the market. The robin egg blue or sky blue color of the Persian turquoise mined near the modern city of Nishapur, Iran, has been used as a guiding reference for evaluating turquoise quality.

Turquoise

General

Category Phosphate minerals

Formula CuAl6(PO4)4(OH)8•4H2O

IMA symbol Tqu

Strunz classification 8.DD.15

Crystal system Triclinic

Crystal class Pinacoidal (1)

(same H–M symbol)

Identification

Colour Turquoise, blue, blue-green, green

Crystal habit Massive, nodular

Cleavage Perfect on {001}, good on {010}, but cleavage rarely seen

Fracture Conchoidal

Mohs scale hardness 5–6

Luster Waxy to subvitreous

Streak Bluish white

Diaphaneity Opaque

Specific gravity 2.6–2.9

Optical properties Biaxial (+)

Refractive index nα = 1.610

nβ = 1.615

nγ = 1.650

Birefringence +0.040

Pleochroism Weak

Fusibility Fusible in heated HCl

Solubility Soluble in HCl

Names

The word turquoise dates to the 17th century and is derived from the Old French turquois meaning "Turkish" because the mineral was first brought to Europe through the Ottoman Empire. However, according to Etymonline, the word dates to the 14th century with the form turkeis, meaning "Turkish", which was replaced with turqueise from French in the 1560s. According to the same source, the gemstone was first brought to Europe from Turkestan or another Turkic territory. Pliny the Elder referred to the mineral as callais (from Ancient Greek κάλαϊς) and the Aztecs knew it as chalchihuitl.

In professional mineralogy, until the mid-19th century, the scientific names kalaite: 571–572  or azure spar were also used, which simultaneously provided a version of the mineral origin of turquoise.: 452  However, these terms did not become widespread and gradually fell out of use.

Properties

The finest of turquoise reaches a maximum Mohs hardness of just under 6, or slightly more than window glass. Characteristically a cryptocrystalline mineral, turquoise almost never forms single crystals, and all of its properties are highly variable. X-ray diffraction testing shows its crystal system to be triclinic. With lower hardness comes greater porosity. The lustre of turquoise is typically waxy to subvitreous, and its transparency is usually opaque, but may be semitranslucent in thin sections. Colour is as variable as the mineral's other properties, ranging from white to a powder blue to a sky blue and from a blue-green to a yellowish green. The blue is attributed to idiochromatic copper while the green may be the result of iron impurities (replacing copper.): 29 

The refractive index of turquoise varies from 1.61 to 1.65 on the three crystal axes, with birefringence 0.040, biaxial positive, as measured from rare single crystals.

Turquoises are rarely pure, blue-green in color. They are more commonly found dotted with small spots or interspersed with brown, gray, or black, cobweb-like veins, the so-called matrix (correspondingly, turquoise matrix or matrix turquoise). This consists either of other minerals such as pyrite or of host rocks such as limonite and others.

Crushed turquoise is soluble in hot hydrochloric acid. Its streak is white to greenish to blue, and its fracture is smooth to conchoidal. Despite its low hardness relative to other gems, turquoise takes a good polish. Turquoise may also be peppered with flecks of pyrite or interspersed with dark, spidery limonite veining.

Turquoise is nearly always cryptocrystalline and massive and assumes no definite external shape. Crystals, even at the microscopic scale, are rare. Typically the form is a vein or fracture filling, nodular, or botryoidal in habit. Stalactite forms have been reported. Turquoise may also pseudomorphously replace feldspar, apatite, other minerals, or even fossils. Odontolite is fossil bone or ivory that has historically been thought to have been altered by turquoise or similar phosphate minerals such as the iron phosphate vivianite. Intergrowth with other secondary copper minerals such as chrysocolla is also common. Turquoise is distinguished from chrysocolla, the only common mineral with similar properties, by its greater hardness.

Turquoise forms a complete solid solution series with chalcosiderite, CuFe6(PO4)4(OH)8•4H2O, in which ferric iron replaces aluminium.

Hydrochloric acid only attacks turquoise when heated. Organic acids such as formic, citric, or acetic acid, however, affect all minerals in the turquoise group. Potassium hydroxide also decomposes turquoise. Although the mineral doesn't melt in the presence of a blowtorch, it disintegrates into a black powder at temperatures between 200 and 600 °C, crackling with a loud noise.

Formation

Turquoise deposits probably form in more than one way. However, a typical turquoise deposit begins with hydrothermal deposition of copper sulfides. This takes place when hydrothermal fluids leach copper from a host rock, which is typically an intrusion of calc-alkaline rock with a moderate to high silica content that is relatively oxidized. The copper is redeposited in more concentrated form as a copper porphyry, in which veins of copper sulfide fill joints and fractures in the rock. Deposition takes place mostly in the potassic alteration zone, which is characterized by conversion of existing feldspar to potassium feldspar and deposition of quartz and micas at a temperature of 400–600 °C (752–1,112 °F)

Turquoise is a secondary or supergene mineral, not present in the original copper porphyry. It forms when meteoric water (rain or snow melt infiltrating the Earth's surface) percolates through the copper porphyry. Dissolved oxygen in the water oxidizes the copper sulfides to soluble sulfates, and the acidic, copper-laden solution then reacts with aluminum and potassium minerals in the host rock to precipitate turquoise. This typically fills veins in volcanic rock or phosphate-rich sediments. Deposition usually takes place at a relatively low temperature, 90–195 °C (194–383 °F), and seems to occur more readily in arid environments.

Turquoise in the Sinai Peninsula is found in lower Carboniferous sandstones overlain by basalt flows and upper Carboniferous limestone. The overlying beds were presumably the source of the copper, which precipitated as turquoise in nodules, horizontal seams, or vertical joints in the sandstone beds. The classical Iranian deposits are found in sandstones and limestones of Tertiary age were intruded by apatite-rich porphyritic trachytes and mafic rock. Supergene alteration fractured the rock and converted some of the minerals in the rock to alunite, which freed aluminum and phosphate to combine with copper from oxidized copper sulfides to form turquoise. This process took place at a relatively shallow depth, and by 1965 the mines had "bottomed" at a depth averaging just 9 meters (30 ft) below the surface.

Turquoise deposits are widespread in North America. Some deposits, such as those of Saguache and Conejos Counties in Colorado or the Cerrillos Hills in New Mexico, are typical supergene deposits formed from copper porphyries. The deposits in Cochise County, Arizona, are found in Cambrian quartzites and geologically young granites and go down at least as deep as 54 meters (177 ft).

Classification

Both the mineral classification according to Strunz and the system according to Dana, which is commonly used in English-speaking countries, classify turquoise in the mineral class of “phosphates, arsenates and vanadates”.

In the outdated, but still partly in use 8th edition of Strunz's system, turquoise belonged to the section of " hydrous phosphates with foreign anions ", where it formed the name giver of the "turquoise group" with the system number VII/D.15 and the other members aheylite, chalcosiderite, faustite and planerite.

The 9th edition of Strunz's mineral classification, valid since 2001, retains the same division and group name, but the division is now more precisely subdivided according to the size of the cations involved and the molar ratio between the foreign anion and the sulfate, arsenate, or vanadate complex. According to its composition, turquoise belongs to the subdivision "Containing exclusively medium-sized cations; (OH, etc.): RO 4 = 2: 1" and, together with aheylite, chalcosiderite, faustite, and planerite, still forms the "turquoise group" with the system number 8.DD.15.

In the Dana classification of minerals, which is primarily sorted according to the crystal system, turquoise is found in the section of “ hydrous phosphates etc., with hydroxyl or halogen with (A) 3 (XO 4) 2 Z q • x(H 2 O) ” and there as a triclinic crystallizing mineral in the “turquoise group” with the system number 42.09.03.

Crystal structure

Considering the millennia in which turquoise has been known worldwide and valued as a gemstone, its crystal structure has been elucidated unusually late. Such structural analyses are based on the evaluation of X-ray diffraction experiments on single crystals of sufficient size and quality. However, turquoise has always been known only in the form of earthy, cryptocrystalline masses. Single crystals of turquoise were first described in 1912 from a deposit in Virginia (USA), and it was not until 1965 that the turquoise structure was fully elucidated using single crystals from this locality. 

Turquoise crystallizes in the triclinic crystal system in space group P1 (space group no. 2). The only symmetry element is an inversion center, which multiplies the atoms through point reflection. Because copper coincides in position with the inversion center, it is the only particle that appears only once in the chemical formula. In crystallography, the coincidence of particles with a symmetry element is referred to as a special position. All other atoms are in symmetry-free, so-called general positions. The lattice parameters of the unit cell are given in the table.

The cations are four- and six-fold coordinated by oxygen in the crystal structure. The two crystallographically distinct phosphorus cations (P 5+) in the crystal lattice are coordinated as single particles by only four oxygen atoms in the shape of a tetrahedron. These 3− tetrahedra are not interconnected but exist isolated in the structure. Each phosphorus ion is bonded via two oxygen atoms to two Al ions at position Al-3, one Al ion at Al-1, and another at Al-2.

Aluminum (Al 3+) and the small amounts of Fe 3+ ions are located at three different positions, each octahedrally surrounded by six oxygen atoms. Al at positions Al-1 and Al-2 is coordinated by two oxygen atoms, three OH groups, and one H 2 O molecule. Al at position Al-3 is surrounded by four oxygen atoms and two OH groups.

Copper is located at an inversion center at the corners of the unit cell and is arranged in a distorted octahedral configuration surrounded by four OH groups and two H2O molecules. This highly distorted octahedron is connected via shared edges to four octahedra, two of which are in turn connected via a shared edge. This group of five linked octahedra within the crystal structure can be referred to as a cluster and, for the sake of simplicity of describing the structure, can be considered as a single building block.

The clusters of edge-sharing and octahedra are connected to each other via another AlO 6 octahedron and the PO 4 tetrahedra. This third octahedron is connected to the tetrahedra and the Cu-Al octahedral clusters via shared corners, i.e., shared oxygen atoms.

For simplicity, the structural images do not show the atoms and their bonds, but rather the coordination polyhedra (tetrahedra and octahedra). The cations Cu 2+, Al 3+, and P 5+ are located approximately in the center of the polyhedra, and the oxygen atoms bonded to them are located at the polyhedron vertices. Also not shown are the hydrogen atoms of the OH groups and H 2 O molecules. Their oxygen atoms contribute to the octahedral coordination of the copper and aluminum. The blue lines in the second image mark the edges of the unit cell.

Varieties

Both henwoodite (first described in 1876 by Collins) and rashleighite (iron turquoise, first described in 1948 by Russell)  are classified as a low-iron variety of turquoise. Other sources consider the term henwoodite to be a synonym for turquoise. 

A continuous solid solution series exists between the predominantly blue turquoise and the predominantly green chalcosiderite, with the aluminum-rich compounds corresponding to turquoise and the iron-rich compounds being assigned to chalcosiderite. The color spectrum is therefore as diverse as the properties of the mineral - it ranges from white to light blue to deep blue and can be both green-blue and yellow-blue. The blue color is attributed to the idiochromatic copper, while the green is the result of the admixture of iron, which has replaced the aluminum. Other, small admixtures of, for example, calcium can also cause variation in color.

Due to dehydration (drying out), which occurs shortly after the mineral is mined outside the mine, the turquoise loses its color intensity and becomes lighter.

Agapit (agaphite) and johnite, on the other hand, are names for turquoise with a glassy surface. 

Occurrence

Turquoise was among the first gems to be mined, and many historic sites have been depleted, though some are still worked to this day. These are all small-scale operations, often seasonal owing to the limited scope and remoteness of the deposits. Most are worked by hand with little or no mechanization. However, turquoise is often recovered as a byproduct of large-scale copper mining operations, especially in the United States.

Deposits typically take the form of small veins in partially decomposed volcanic rock in arid climates.

Iran

Iran has been an important source of turquoise for at least 2,000 years. It was initially named by Iranians "pērōzah" meaning "victory", and later the Arabs called it "fayrūzah", which is pronounced in Modern Persian as "fīrūzeh". In Iranian architecture, the blue turquoise was used to cover the domes of palaces because its intense blue colour was also a symbol of heaven on earth.

This deposit is blue naturally and turns green when heated due to dehydration. It is restricted to a mine-riddled region in Nishapur, the 2,012 m (6,601 ft) mountain peak of Ali-mersai near Mashhad, the capital of Khorasan Province, Iran. Weathered and broken trachyte is host to the turquoise, which is found both in situ between layers of limonite and sandstone and amongst the scree at the mountain's base. These workings are the oldest known, together with those of the Sinai Peninsula. Iran also has turquoise mines in Semnan and Kerman provinces.

Sinai

Since at least the First Dynasty (3000 BCE) in ancient Egypt, and possibly before then, turquoise was used by the Egyptians and was mined by them in the Sinai Peninsula. This region was known as the Country of Turquoise by the native Monitu. There are six mines in the peninsula, all on its southwest coast, covering an area of some 650 km2 (250 sq mi). The two most important of these mines, from a historical perspective, are Serabit el-Khadim and Wadi Maghareh, believed to be among the oldest of known mines. The former mine is situated about 4 kilometres from an ancient temple dedicated to the deity Hathor.

The turquoise is found in sandstone that is, or was originally, overlain by basalt. Copper and iron workings are present in the area. Large-scale turquoise mining is not profitable today, but the deposits are sporadically quarried by Bedouin peoples using homemade gunpowder. In the rainy winter months, miners face a risk from flash flooding; even in the dry season, death from the collapse of the haphazardly exploited sandstone mine walls may occur. The colour of Sinai material is typically greener than that of Iranian material but is thought to be stable and fairly durable. Often referred to as "Egyptian turquoise", Sinai material is typically the most translucent, and under magnification, its surface structure is revealed to be peppered with dark blue discs not seen in material from other localities.

United States

The Southwest United States is a significant source of turquoise; Arizona, California (San Bernardino, Imperial, Inyo counties), Colorado (Conejos, El Paso, Lake, Saguache counties), New Mexico (Eddy, Grant, Otero, Santa Fe counties) and Nevada (Clark, Elko, Esmeralda County, Eureka, Lander, Mineral County and Nye counties) are (or were) especially rich. The deposits of California and New Mexico were mined by pre-Columbian Native Americans using stone tools, some local and some from as far away as central Mexico. Cerrillos, New Mexico is thought to be the location of the oldest mines; prior to the 1920s, the state was the country's largest producer; it is more or less exhausted today. Only one mine in California, located at Apache Canyon, operates at a commercial capacity today.

The turquoise occurs as vein or seam fillings, and as compact nuggets; these are mostly small in size. While quite fine material is sometimes found, rivalling Iranian material in both colour and durability, most American turquoise is of a low grade (called "chalk turquoise"); high iron levels mean greens and yellows predominate, and a typically friable consistency in the turquoise's untreated state precludes use in jewelry.

Arizona is currently the most important producer of turquoise by value. Several mines exist in the state, two of them famous for their unique colour and quality and considered the best in the industry: the Sleeping Beauty Mine in Globe ceased turquoise mining in August 2012. The mine chose to send all ore to the crusher and to concentrate on copper production due to the rising price of copper on the world market. The price of natural untreated Sleeping Beauty turquoise has risen dramatically since the mine's closing. The Kingman Mine as of 2015 still operates alongside a copper mine outside of the city. Other mines include the Blue Bird mine, Castle Dome, and Ithaca Peak, but they are mostly inactive due to the high cost of operations and federal regulations. The Phelps Dodge Lavender Pit mine at Bisbee ceased operations in 1974 and never had a turquoise contractor. All Bisbee turquoise was "lunch pail" mined. It came out of the copper ore mine in miners' lunch pails. Morenci and Turquoise Peak are either inactive or depleted.

Nevada is the country's other major producer, with more than 120 mines which have yielded significant quantities of turquoise. Unlike elsewhere in the US, most Nevada mines have been worked primarily for their gem turquoise and very little has been recovered as a byproduct of other mining operations. Nevada turquoise is found as nuggets, fracture fillings and in breccias as the cement filling interstices between fragments. Because of the geology of the Nevada deposits, a majority of the material produced is hard and dense, being of sufficient quality that no treatment or enhancement is required. While nearly every county in the state has yielded some turquoise, the chief producers are in Lander and Esmeralda counties. Most of the turquoise deposits in Nevada occur along a wide belt of tectonic activity that coincides with the state's zone of thrust faulting. It strikes at a bearing of about 15° and extends from the northern part of Elko County, southward down to the California border southwest of Tonopah. Nevada has produced a wide diversity of colours and mixes of different matrix patterns, with turquoise from Nevada coming in various shades of blue, blue-green, and green. Some of this unusually-coloured turquoise may contain significant zinc and iron, which is the cause of the beautiful bright green to yellow-green shades. Some of the green to green-yellow shades may actually be variscite or faustite, which are secondary phosphate minerals similar in appearance to turquoise. A significant portion of the Nevada material is also noted for its often attractive brown or black limonite veining, producing what is called "spiderweb matrix". While a number of the Nevada deposits were first worked by Native Americans, the total Nevada turquoise production since the 1870s has been estimated at more than 600 short tons (540 t), including nearly 400 short tons (360 t) from the Carico Lake mine. In spite of increased costs, small scale mining operations continue at a number of turquoise properties in Nevada, including the Godber, Orvil Jack and Carico Lake mines in Lander County, the Pilot Mountain Mine in Mineral County, and several properties in the Royston and Candelaria areas of Esmerelda County.

In 1912, the first deposit of distinct, single-crystal turquoise was discovered at Lynch Station in Campbell County, Virginia. The crystals, forming a druse over the mother rock, are very small; 1 mm (0.04 in) is considered large. Until the 1980s Virginia was widely thought to be the only source of distinct crystals; there are now at least 27 other localities.

In an attempt to recoup profits and meet demand, some American turquoise is treated or enhanced to a certain degree. These treatments include innocuous waxing and more controversial procedures, such as dyeing and impregnation. There are some American mines which produce materials of high enough quality that no treatment or alterations are required. Any such treatments which have been performed should be disclosed to the buyer on sale of the material.

Other sources

Turquoise prehistoric artifacts (beads) are known since the fifth millennium BCE from sites in the Eastern Rhodopes in Bulgaria – the source for the raw material is possibly related to the nearby Spahievo lead–zinc ore field. In Spain, turquoise has been found as a minor mineral in the variscite deposits exploited during prehistoric times in Palazuelos de las Cuevas (Zamora) and in Can Tintorer, Gavá (Barcelona).

China has been a minor source of turquoise for 3,000 years or more. Gem-quality material, in the form of compact nodules, is found in the fractured, silicified limestone of Yunxian and Zhushan, Hubei province. Additionally, Marco Polo reported turquoise found in present-day Sichuan. Most Chinese material is exported, but a few carvings worked in a manner similar to jade exist. In Tibet, gem-quality deposits purportedly exist in the mountains of Derge and Nagari-Khorsum in the east and west of the region respectively.

Other notable localities include: Afghanistan; Australia (Victoria and Queensland); north India; northern Chile (Chuquicamata); Cornwall; Saxony; Silesia; and Turkestan.

Use as a gemstone

Turquoise is one of the oldest gemstones and, with its wide range of green-blue tones from delicate pastel to deep luminosity, has captivated many peoples since ancient times. It adorned the rulers of ancient Egypt, the Aztecs (and probably also the pre-Columbian Mesoamericans), the Persians and Mesopotamians, as well as nobles in the Indus and, to some extent, in ancient China since the last Shang Dynasty. Turquoise first reached Europe with traders on the Silk Road. During the Biedermeier period, the sky-blue color variations were particularly popular. 

However, it has only been used in the jewelry industry since the 14th century, as this period marked the decline of the Catholic Church, which had previously used it for ecclesiastical jewelry. It was unknown in India until the Mughal period and in Japan until the 18th century. Turquoise was believed by many of these cultures to have prophylactic properties. It was said to change color depending on the wearer's state of health and provide protection against evil forces.

Today, turquoise is most commonly found in the West as a cabochon in silver rings, Native American-style bracelets, or as crudely crafted beads in necklaces. To a lesser extent, turquoise is also used by the Zuni for fetish carvings. Deep blue tones are still considered valuable, but greenish or yellow pieces are very popular with artists. In Western culture, turquoise is the traditional birthstone for those born in December.

Engraved in turquoise

The inconsistent hardness of turquoise must have prevented the ancients from frequently carving on this stone, while ancient specimens must have been altered in their history. Be that as it may, few turquoise carvings are known. However, there are a few:

Anamulet from the Genevosiocollection, convex on one side and flat on the other, representing on one sideDianawith two branches in her hands and on the other a kind ofsistrum, a star and a bee, with Greek words on both sides

The Duke of Orleans'scabinet had two engraved turquoises, one representing Diana with her quiver on her back and the other Faustina, the mother

There is also a turquoise piece in the gallery ofFlorence, the size of a billiard ball, on which a head is engraved. It was thought to representCaesar's head, but it seems to representTiberius 

History of use

The pastel shades of turquoise have endeared it to many great cultures of antiquity: it has adorned the rulers of Ancient Egypt, the Aztecs (and possibly other Pre-Columbian Mesoamericans), Persia, Mesopotamia, the Indus Valley, and to some extent in ancient China since at least the Shang dynasty. Despite being one of the oldest gems, probably first introduced to Europe (through Turkey) with other Silk Road novelties, turquoise did not become important as an ornamental stone in the West until the 14th century, following a decline in the Roman Catholic Church's influence which allowed the use of turquoise in secular jewellery. It was apparently unknown in India until the Mughal period, and unknown in Japan until the 18th century. A common belief shared by many of these civilizations held that turquoise possessed certain prophylactic qualities; it was thought to change colour with the wearer's health and protect him or her from untoward forces.

The Aztecs viewed turquoise as an embodiment of fire and gave it properties such as heat and smokiness. They inlaid turquoise, together with gold, quartz, malachite, jet, jade, coral, and shells, into provocative (and presumably ceremonial[clarification needed]) mosaic objects such as masks (some with a human skull as their base), knives, and shields. Natural resins, bitumen and wax were used to bond the turquoise to the objects' base material; this was usually wood, but bone and shell were also used. Like the Aztecs, the Pueblo, Navajo and Apache tribes cherished turquoise for its amuletic use; the latter tribe believe the stone to afford the archer dead aim. In Navajo culture it is used for "a spiritual protection and blessing." Among these peoples turquoise was used in mosaic inlay, in sculptural works, and was fashioned into toroidal beads and freeform pendants. The Ancestral Puebloans (Anasazi) of the Chaco Canyon and surrounding region are believed to have prospered greatly from their production and trading of turquoise objects. The distinctive silver jewellery produced by the Navajo and other Southwestern Native American tribes today is a rather modern development, thought to date from around 1880 as a result of European influences.

In Persia, turquoise was the de facto national stone for millennia, extensively used to decorate objects (from turbans to bridles), mosques, and other important buildings both inside and out, such as the Medresseh-i Shah Husein Mosque of Isfahan. The Persian style and use of turquoise was later brought to India following the establishment of the Mughal Empire there, its influence seen in high purity gold jewellery (together with ruby and diamond) and in such buildings as the Taj Mahal. Persian turquoise was often engraved with devotional words in Arabic script which was then inlaid with gold.

Cabochons of imported turquoise, along with coral, was (and still is) used extensively in the silver and gold jewellery of Tibet and Mongolia, where a greener hue is said to be preferred. Most of the pieces made today, with turquoise usually roughly polished into irregular cabochons set simply in silver, are meant for inexpensive export to Western markets and are probably not accurate representations of the original style.

The Ancient Egyptian use of turquoise stretches back as far as the First Dynasty and possibly earlier; however, probably the most well-known pieces incorporating the gem are those recovered from Tutankhamun's tomb, most notably the Pharaoh's iconic burial mask which was liberally inlaid with the stone. It also adorned rings and great sweeping necklaces called pectorals. Set in gold, the gem was fashioned into beads, used as inlay, and often carved in a scarab motif, accompanied by carnelian, lapis lazuli, and in later pieces, coloured glass. Turquoise, associated with the goddess Hathor, was so liked by the Ancient Egyptians that it became (arguably) the first gemstone to be imitated, the fair structure created by an artificial glazed ceramic product known as faience.

The French conducted archaeological excavations of Egypt from the mid-19th century through the early 20th. These excavations, including that of Tutankhamun's tomb, created great public interest in the western world, subsequently influencing jewellery, architecture, and art of the time. Turquoise, already favoured for its pastel shades since around 1810, was a staple of Egyptian Revival pieces. In contemporary Western use, turquoise is most often encountered cut en cabochon in silver rings, bracelets, often in the Native American style, or as tumbled or roughly hewn beads in chunky necklaces. Lesser material may be carved into fetishes, such as those crafted by the Zuni. While strong sky blues remain superior in value, mottled green and yellowish material is popular with artisans.

Egypt

Grave goods prove that the ancient Egyptians used turquoise as a gemstone since the Predynastic Period (around 5500 BC). The most famous pieces, however, probably come from Tutankhamun's tomb. The pharaoh 's death mask, which was generously decorated with turquoise, is particularly well known. Egyptian goldsmiths also used it for rings, lavish necklaces and pectorals. Turquoise can be found in gold braid and as a material for amulet carvings (scarabs), which were further decorated with carnelian, lapis lazuli and later with colored glass. Turquoise was the gemstone of the Egyptian goddess Hathor and was so sought after by the ancient Egyptians that it was one of the first gemstones to be imitated. To produce this lighter material, the ceramic product faience is glazed. A similarly blue "pottery" from the Bronze Age has been discovered in a grave in the British Isles.

The French conducted archaeological excavations in Egypt from the mid-19th to the early 20th century. These excavations, which included the tomb of Tutankhamun, aroused great interest in the Western world and thus influenced the jewelry, architecture, and art of the time. Turquoise, which had been sought after for its color since 1810, became a hallmark of Egyptian Revival pieces.

The goddess Hathor was associated with turquoise, as she was the patroness of Serabit el-Khadim, where it was mined. Her titles included "Lady of Turquoise", "Mistress of Turquoise", and "Lady of Turquoise Country".

In Western culture, turquoise is also the traditional birthstone for those born in the month of December. The turquoise is also a stone in the Jewish High Priest's breastplate, described in Exodus chapter 28. The stone is also considered sacred to the indigenous Zuni and Pueblo peoples of the American Southwest. The pre-Columbian Aztec and Maya also considered it to be a valuable and culturally important stone.

The Egyptians were the first to produce an artificial imitation of turquoise, in the glazed earthenware product faience. Later glass and enamel were also used, and in modern times more sophisticated porcelain, plastics, and various assembled, pressed, bonded, and sintered products (composed of various copper and aluminium compounds) have been developed: examples of the latter include "Viennese turquoise", made from precipitated aluminium phosphate coloured by copper oleate; and "neolith", a mixture of bayerite and copper(II) phosphate. Most of these products differ markedly from natural turquoise in both physical and chemical properties, but in 1972 Pierre Gilson introduced one fairly close to a true synthetic (it does differ in chemical composition owing to a binder used, meaning it is best described as a simulant rather than a synthetic). Gilson turquoise is made in both a uniform colour and with black "spiderweb matrix" veining not unlike the natural Nevada material.

The most common imitation of turquoise encountered today is dyed howlite and magnesite, both white in their natural states, and the former also having natural (and convincing) black veining similar to that of turquoise. Dyed chalcedony, jasper, and marble is less common, and much less convincing. Other natural materials occasionally confused with or used in lieu of turquoise include: variscite and faustite; chrysocolla (especially when impregnating quartz); lazulite; smithsonite; hemimorphite; wardite; and a fossil bone or tooth called odontolite or "bone turquoise", coloured blue naturally by the mineral vivianite. While rarely encountered today, odontolite was once mined in large quantities—specifically for its use as a substitute for turquoise—in southern France.

These fakes are detected by gemologists using a number of tests, relying primarily on non-destructive, close examination of surface structure under magnification; a featureless, pale blue background peppered by flecks or spots of whitish material is the typical surface appearance of natural turquoise, while manufactured imitations will appear radically different in both colour (usually a uniform dark blue) and texture (usually granular or sugary). Glass and plastic will have a much greater translucency, with bubbles or flow lines often visible just below the surface. Staining between grain boundaries may be visible in dyed imitations.

Some destructive tests may be necessary; for example, the application of diluted hydrochloric acid will cause the carbonates odontolite and magnesite to effervesce and howlite to turn green, while a heated probe may give rise to the pungent smell so indicative of plastic. Differences in specific gravity, refractive index, light absorption (as evident in a material's absorption spectrum), and other physical and optical properties are also considered as means of separation.

China

In Tibet and Mongolia in ancient China, cabochons with imported turquoise and coral were and are widely used in the silver and gold jewelry industry. Greener stones are often preferred. Today, these pieces are often produced for the Western market and are only an inaccurate representation of the original design. The turquoise is roughly polished into irregular cabochons and set in silver.

Aztecs and other American cultures

The Aztecs used turquoise, but also gold, quartz, malachite, jet, jade, coral, and shells to create deterrent and probably ritual objects decorated with mosaics, such as masks, knives, and shields. Wood, bone, and shells could serve as substrates for mosaics, and resins, bitumen, and wax could serve as adhesives.

In addition to the Aztecs, the Pueblo, Diné, and Apache also valued turquoise as a gemstone. They used it for amulets, and the Apache believed it gave them powers to aid archery. These peoples also used turquoise to decorate sculptures, ring-shaped beads, and pendants. The Anasazi of Chaco Canyon and the surrounding areas are said to have become very wealthy through the turquoise trade. The unique silver jewelry of the Navajo and other Southwest American Indian tribes, however, is a more modern phenomenon and is attributed to 19th-century European influences.

Central Asia

In Central Asia, it is considered a valuable talisman symbolizing courage and hope, as well as bringing success to men in love and virtue to girls. Another belief is that it protects the nose and respiratory tract. It is placed under the sign of Taurus.

Persia

In Persia, turquoise has been the national gemstone for thousands of years. In Persian mythology, only the king and his sub-kings were permitted to have throne and crown with turquoise. Turquoise was (later) used to decorate various everyday objects (turbans), mosques and other important buildings such as the Madrassa-I-Shah-Hussein Mosque in Isfahan. During the Mughal Empire, the Persian style and use of turquoise also came to India, where it can be admired in fine gold jewelry (along with rubies and diamonds) and buildings such as the Taj Mahal. Persian turquoise was often engraved with Arabic script and then decorated with gold.

In Bible

In the Luther Bible, in the Book of Exodus, there is a description of the "brace of righteousness" as part of a priestly robe for Aaron (Exodus 28:15–30). The breastplate (choshen) attached to the ephod (priestly apron) was decorated with twelve precious stones set in gold and arranged in four rows. The name of one of the Twelve Tribes of Israel was engraved on each precious stone. Various scholars translate the first and second stones of the third row as turquoise, but others believe the stones to be jacinth (zircon, hyacinth) and agate. Scholars, however, do not agree as to which stone corresponds to which tribe.

In Shamanism and esotericism

Among the so-called "Native Americans", such as the Aztecs, Hohokam, Moche, Navajo, and Zuñi, turquoise played a prominent, mythical, and spiritual role and was considered a powerful protective and healing stone, which, for example, could protect travelers from harm and, if used correctly by shamans, cure illnesses. The Aztec rain god (Tlaloc) was appeased by gifts of turquoise, and Quetzalcoatl was responsible for the craftsmanship of the ancient Mexican stone carvers ("lapidaries"). 

In China, turquoise was less important than jade, but it was still considered a lucky and protective stone that could bring joy, health, and long life. In Egypt and Persia, it was also said to have strong protective properties, and the deceased received it as a grave good in the form of jewelry and amulets.

In modern esotericism, turquoise is used as a healing stone for various ailments, usually in combination with others. For example, it is said to help with inflammatory diseases when worn on the body (as a necklace or bracelet) or when water containing turquoise is drunk. There is no scientific evidence for this.

Various astrologers associate turquoise with Mercury (according to Raphael, 1987), Venus (according to Ahlborn, 1996), Jupiter (according to Richardson and Huett, 1989), or Uranus (according to Uyldert, 1983). As a zodiac stone, it is associated with Aquarius, among others, and as a monthly stone, December. 

Appreciation and care

The value of turquoise as a gemstone is generally determined by its color intensity, with a deep sky blue being the most desirable. The greener, lighter, or more mottled the stone, the less valuable it is. However, as mentioned above, a green color is preferred in East Asian countries.

Regardless of the color of the stone, it shouldn't be too soft or chalky, even if it has been treated. This material tends to fade over time and doesn't meet the normal quality standards of the jewelry industry. The best turquoise is usually found in arid regions.

The aforementioned cobweb matrix can increase a stone's value. This shape is very popular in the Southwestern United States and Asia, but is frowned upon in the Middle East, where pure, blue turquoise is preferred. Uniform coloration is desirable, and in worked pieces, craftsmanship is also important. This includes the quality of polish and the symmetry of the stone. Calibrated stones—those that meet jewelry industry standards—are preferred. As with coral, the price of turquoise is usually determined by its size in millimeters, not its weight.

Turquoise is treated in a variety of ways, some more permanent and radical than others. Experts disagree on whether some of these methods are acceptable, but generally, "light" waxing or oiling to enhance the color and luster of the turquoise is "permitted." This is provided the original mineral is of very high quality and very little wax or oil is absorbed, meaning the stone does not require regular care to maintain its beauty. In general, however, untreated turquoise is always more expensive than treated or synthetic turquoise.

As a phosphate mineral, turquoise is very sensitive to acidic or alkaline solutions. Perspiration, perfumes, skin oils, and other cosmetics, as well as cleaning agents such as soap, damage the stone. Turquoise jewelry should therefore be removed when washing or cleaning hands, as its color can turn an unsightly, brownish-green over time. Turquoise is also sensitive to heat. A temperature of around 250 °C, which can easily be reached during soldering or polishing, causes the stone to take on a green color. If the stone is exposed to direct sunlight for a long time, it loses color and/or crystal water (it dehydrates). Therefore, when wearing turquoise jewelry, care should be taken to apply cosmetics, sunscreen, and hairspray before putting on the jewelry. It should not be worn on the beach or while sunbathing. To prevent deposits, it can be gently cleaned with a soft cloth after wearing. A separate box is suitable for storage to avoid scratching from other gemstones.

Manipulations and imitations

Turquoise is treated to enhance both its colour and durability (increased hardness and decreased porosity). As is so often the case with any precious stones, full disclosure about treatment is frequently not given. Gemologists can detect these treatments using a variety of testing methods, some of which are destructive, such as the use of a heated probe applied to an inconspicuous spot, which will reveal oil, wax or plastic treatment.

The Egyptians appear to have been the first to be able to produce artificial turquoise using the glazed clay product Egyptian faience. Later, glass and enamel were also used, and in modern times, more sophisticated ceramic products, porcelain, plastic, and other reconstructed, pressed, glued, and fired raw materials for the production of artificial turquoise have emerged. The latter consist of various copper and aluminum components.

Examples include "Viennese turquoise," made from precipitated aluminum orthophosphate and colored with copper oleates, and "Neolith" (Reese turquoise), a mixture of bayerite and copper phosphates. Both products differ significantly from the original in terms of physical and chemical properties. Another turquoise imitation is known as "Neoturquoise," and is produced using gibbsite and copper phosphate.

In 1972, Pierre Gilson succeeded in producing something resembling synthetic turquoise. However, due to the adhesive used, its chemical composition differs, so it's better described as an imitation rather than synthetic. Gilson's turquoise is available in a uniform color and with the black "spiderweb" matrix (spiderweb turquoise), which is not dissimilar to Nevada turquoise.

Due to the widespread use of artificially treated, imitated, or synthetically produced materials, turquoise's popularity has recently declined. Even experts often cannot distinguish such pieces from genuine natural stones.

Due to similar coloration or similar appearance, which is enhanced by artificial coloring, turquoise can be confused with many minerals or mineral intergrowths. Similar minerals include amazonite, chrysocolla, faustite, hemimorphite, lazulite, serpentine, smithsonite, utahite, variscite, and an intergrowth of variscite, chalcedony, and quartz known as amatrix (American matrix). Amatrix, however, is easily distinguished from turquoise due to its Mohs hardness of 7. 

The most common imitation bases are howlite (hardness 3 to 3.5) and magnesite (hardness 4), both of which are white in their original form, but when dyed blue they look very similar to the sought-after matrix turquoise. Dyed chalcedony, jasper, and marble are less common and do not look as authentic. Another common imitation is odontolith, or "bone turquoise," a fossil bone colored using the mineral vivianite. Odontolith was once mined extensively in southern France specifically for turquoise production, but is now largely out of fashion.

All of these fakes can be identified by gemologists through numerous tests, which rely heavily on a thorough examination of the surface structure under a microscope, which must not damage the stone. Natural turquoise has a light blue background dotted with white spots or dots. Man-made stones, on the other hand, are fundamentally different in terms of color (usually a solid dark blue) and surface texture (granular, similar to sugar). Glass and plastics are more translucent, sometimes with bubbles or small streaks below the surface. Spots between the grain boundaries are visible in dyed imitations.

In some cases, however, the stone is destroyed. Dilute hydrochloric acid dissolves the carbonates odontolith and magnesite, forming bubbles, and howlite turns green. Heating the stone produces a pungent odor in plastic products. Originals and fakes can also be distinguished by their density, refractive index, light absorption, and other physical and optical properties. Artificial turquoise is so common today that it has long since surpassed natural turquoise in terms of quantity. It can now be assumed that over 90% of the turquoise sold is treated, reconstructed, or imitated in some way. Even in "authentic" Native American and Chinese Tibetan jewelry, one often only finds artificial or, at best, heavily treated turquoise.

Methods for improvement

Natural turquoise is rarely hard and durable enough to be used in jewelry in its untreated form. Turquoise is therefore improved in various ways before and after cutting. The first methods of improvement were light waxing and oiling, which can increase the luster and intensify the color. These methods are now accepted as tradition, as the raw material is usually of higher quality anyway. Modern treatment methods such as pressure impregnation of otherwise unsaleable calcareous American turquoise with epoxy resin, polystyrene and water glass (alkali silicates) are met with resistance. They are viewed as too radical an intervention in nature. Plastic and alkali silicate are technically superior to oil and wax in terms of durability and stability, as they can also be applied to turquoise that would be too brittle for the older methods. Such treated turquoise is called "reconstructed" or "stabilized" turquoise. The epoxy resin method was invented in the 1950s by Colbaugh Processing of Arizona. The majority of American turquoise is now treated in this way, although the process itself is very expensive and takes several months. Without impregnation, most American mines would be uncompetitive.

Oiled and wax-treated stones tend to "sweat" under low heat or excessive sunlight. Their surface becomes white or cloudy over time. The use of Prussian Blue and other dyes—often in conjunction with adhesive treatments—to intensify, unify, or completely change the color is considered fraudulent, and not only by purists. Furthermore, some color additives fade over time or stain the substrate.

Dyeing is also used to intensify the dark streaks in the turquoise. The most radical method is certainly "reconstruction," in which pieces of broken stone that are too small to be processed in any other way are simply glued together to form a larger stone. Many, if not all, of these replicas no longer contain any natural components or have been filled with foreign minerals (see imitations). Another treatment method—for which no detailed information is known—is the so-called Zachery process, named after its inventor, the electrical engineer and turquoise dealer James E. Zachery. This process supposedly uses only mediocre stones. The turquoise is harder after treatment and has a more beautiful color and luster.

Since high-quality turquoise is usually found only in thin cracks, it is bonded with harder material for strength. The result is a doublet, which is used in certain jewelry designs. Sometimes the turquoise's parent rock is used as a base. Like all of the aforementioned methods, doublets are legal as long as the buyer is informed of this before purchase.

As is often the case with gemstones, however, this is precisely not the case, which is why gemologists often have to examine the suspect stones. By heating, it's relatively easy to determine whether the stone has been treated with oil, wax, or plastic.

Waxing and oiling

Historically, light waxing and oiling were the first treatments used in ancient times, providing a wetting effect, thereby enhancing the colour and lustre. This treatment is more or less acceptable by tradition, especially because treated turquoise is usually of a higher grade to begin with. Oiled and waxed stones are prone to "sweating" under even gentle heat or if exposed to too much sun, and they may develop a white surface film or bloom over time. (With some skill, oil and wax treatments can be restored.)

Backing

Since finer turquoise is often found as thin seams, it may be glued to a base of stronger foreign material for reinforcement. These stones are termed "backed", and it is standard practice that all thinly cut turquoise in the Southwestern United States is backed. Native indigenous peoples of this region, because of their considerable use and wearing of turquoise, have found that backing increases the durability of thinly cut slabs and cabochons of turquoise. They observe that if the stone is not backed it will often crack. Backing of turquoise is not widely known outside of the Native American and Southwestern United States jewellery trade. Backing does not diminish the value of high quality turquoise, and indeed the process is expected for most thinly cut American commercial gemstones.

Zachery treatment

A proprietary process was created by electrical engineer and turquoise dealer James E. Zachery in the 1980s to improve the stability of medium to high-grade turquoise. The process can be applied in several ways: either through deep penetration on rough turquoise to decrease porosity, by shallow treatment of finished turquoise to enhance color, or both. The treatment can enhance color and improve the turquoise's ability to take a polish. Such treated turquoise can be distinguished in some cases from natural turquoise, without destruction, by energy-dispersive X-ray spectroscopy, which can detect its elevated potassium levels. In some instances, such as with already high-quality, low-porosity turquoise that is treated only for porosity, the treatment is undetectable.

Dyeing

The use of Prussian blue and other dyes (often in conjunction with bonding treatments) to "enhance” its appearance, make uniform or completely change the colour, is regarded as fraudulent by some purists, especially since some dyes may fade or rub off on the wearer. Dyes have also been used to darken the veins of turquoise.

Stabilization

Material treated with plastic or water glass is termed "bonded" or "stabilized" turquoise. This process consists of pressure impregnation of otherwise unsaleable chalky American material by epoxy and plastics (such as polystyrene) and water glass (sodium silicate) to produce a wetting effect and improve durability. Plastic and water glass treatments are far more permanent and stable than waxing and oiling, and can be applied to material too chemically or physically unstable for oil or wax to provide sufficient improvement. Conversely, stabilization and bonding are rejected by some as too radical an alteration.

The epoxy binding technique was first developed in the 1950s and has been attributed to Colbaugh Processing of Arizona, a company that still operates today.

Reconstitution

Perhaps the most extreme of treatments is "reconstitution", wherein fragments of fine turquoise material, too small to be used individually, are powdered and then bonded with resin to form a solid mass. Very often the material sold as "reconstituted turquoise" is artificial, with little or no natural stone, made entirely from resins and dyes. In the trade reconstituted turquoise is often called "block turquoise" or simply "block".

Valuation and care

Hardness and richness of colour are two of the major factors in determining the value of turquoise; while colour is a matter of individual taste, generally speaking, the most desirable is a strong sky to robin egg blue (in reference to the eggs of the American robin). Whatever the colour, for many applications, turquoise should not be soft or chalky; even if treated, such lesser material (to which most turquoise belongs) is liable to fade or discolour over time and will not hold up to normal use in jewellery.

The mother rock or matrix in which turquoise is found can often be seen as splotches or a network of brown or black veins running through the stone in a netted pattern; this veining may add value to the stone if the result is complementary, but such a result is uncommon. Such material is sometimes described as "spiderweb matrix"; it is most valued in the Southwest United States and Far East, but is not highly appreciated in the Near East where unblemished and vein-free material is ideal (regardless of how complementary the veining may be). Uniformity of colour is desired, and in finished pieces the quality of workmanship is also a factor; this includes the quality of the polish and the symmetry of the stone. Calibrated stones—that is, stones adhering to standard jewellery setting measurements—may also be more sought after. Like coral and other opaque gems, turquoise is commonly sold at a price according to its physical size in millimetres rather than weight.

Turquoise is treated in many different ways, some more permanent and radical than others. Controversy exists as to whether some of these treatments should be acceptable, but one can be more or less forgiven universally: This is the light waxing or oiling applied to most gem turquoise to improve its colour and lustre; if the material is of high quality to begin with, very little of the wax or oil is absorbed and the turquoise therefore does not rely on this impermanent treatment for its beauty. All other factors being equal, untreated turquoise will always command a higher price. Bonded and reconstituted material is worth considerably less.

Being a phosphate mineral, turquoise is inherently fragile and sensitive to solvents; perfume and other cosmetics will attack the finish and may alter the colour of turquoise gems, as will skin oils, as will most commercial jewellery cleaning fluids. Prolonged exposure to direct sunlight may also discolour or dehydrate turquoise. Care should therefore be taken when wearing such jewels: cosmetics, including sunscreen and hair spray, should be applied before putting on turquoise jewellery, and they should not be worn to a beach or other sun-bathed environment. After use, turquoise should be gently cleaned with a soft cloth to avoid a buildup of residue, and should be stored in its own container to avoid scratching by harder gems. Turquoise can also be adversely affected if stored in an airtight container.

Sourced from Wikipedia