Amethyst

Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from α- a-, "not" and μεθύσκω (Ancient Greek) methysko / μεθώ metho (Modern Greek), "intoxicate", a reference to the belief that the stone protected its owner from drunkenness. Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.

Amethyst, a semiprecious stone, is often used in jewelry.

Amethyst

General

Category Tectosilicates, quartz variety

Formula Silica (silicon dioxide, SiO2)

Crystal system Trigonal

Crystal class Trapezohedral (32)

Space group P3221 (no. 154)

Identification

Color Purple, violet, dark purple

Crystal habit 6 sided prism ending in 6 sided pyramid (typical)

Twinning Dauphine law, Brazil law, and Japan law

Cleavage None

Fracture Conchoidal

Mohs scale hardness 7 (lower in impure varieties)

Luster Vitreous/glassy

Streak White

Diaphaneity Transparent to translucent

Specific gravity 2.65 constant; variable in impure varieties

Optical properties Uniaxial (+)

Refractive index nω = 1.543–1.553

nε = 1.552–1.554

Birefringence +0.009 (B-G interval)

Pleochroism Weak to moderate purple/reddish purple

Melting point 1650±75 °C

Solubility Insoluble in common solvents

Other characteristics Piezoelectric

Etymology

The origin of the name, from the Greek ἀμέθυστος (amethystos), "counteracting intoxication," expresses the ancient belief that a wearer of amethyst was immune to the intoxicating effects of wine. Likewise, wine from an amethyst cup was not supposed to make one drunk. Originally, this (superstitious) belief arose from the custom of diluting (red) wine with water, which then took on a reddish-violet (amethyst-colored) hue. At the same time, one could drink considerably more of it without becoming immediately drunk or suffering from a hangover.

Another version of the name suggests that Bacchus, the god of wine, frightened a young girl so badly that she froze into crystal. The god then sighed, and when his breath touched the stone, it turned purple, like the color of wine.

Amethyst was also said to have an apotropaic effect against theft. This fact is documented by prehistoric grave finds. Merovingian graves containing amethyst show, if any, signs of robbery, not in the area where the stones (necklaces) were deposited, even if gold pendants were also found there. It is possible that in this early historical period, amethyst had a more far-reaching (and now irreconstructable) alleged (negative) effect on thieves.

Structure

Amethyst is a violet variety of quartz (SiO2) and owes its violet color to irradiation, impurities of iron (Fe3+) and in some cases other transition metals, and the presence of other trace elements, which result in complex crystal lattice substitutions. The irradiation causes the iron Fe3+ ions that replace Si in the lattice to lose an electron and form a 0 color center. Amethyst is a three-dimensional network of tetrahedra where the silicon atoms are in the center and are surrounded by four oxygen atoms located at the vertices of a tetrahedron. This structure is quite rigid and results in quartz's hardness and resistance to weathering. The hardness of the mineral is the same as quartz, thus making it suitable for use in jewelry.

Hue and tone

Amethyst occurs in primary hues from a light lavender or pale violet to a deep purple. Amethyst may exhibit one or both secondary hues, red and blue. High-quality amethyst can be found in Siberia, Sri Lanka, Brazil, Uruguay, and the Far East. The ideal grade, called "Deep Siberian", has a primary purple hue of around 75–80%, with 15–20% blue and (depending on the light source) red secondary hues. "Rose de France" is defined by its markedly light shade of the purple, reminiscent of a lavender / lilac shade. These pale colors were once considered undesirable, but have recently become popular due to intensive marketing.

Green quartz is sometimes called green amethyst; the scientific name is prasiolite. Other names for green quartz are vermarine and lime citrine.

Amethyst frequently shows color zoning, with the most intense color typically found at the crystal terminations. One of gem cutters' tasks is to make a finished product with even color. Sometimes, only a thin layer of a natural, uncut amethyst is violet colored, or the color is very uneven. The uncut gem may have only a small portion that is suitable for faceting.

The color of amethyst has been demonstrated to result from substitution by irradiation of trivalent iron (Fe3+) for silicon in the structure, in the presence of trace elements of large ionic radius, and to a certain extent, the amethyst color can naturally result from displacement of transition elements even if the iron concentration is low. Natural amethyst is dichroic in reddish violet and bluish violet, but when heated, turns yellow-orange, yellow-brown, or dark brownish and may resemble citrine, but loses its dichroism, unlike genuine citrine. When partially heated, amethyst can result in ametrine.

Amethyst can fade in tone if overexposed to light sources, and can be artificially darkened with adequate irradiation. It does not fluoresce under either short-wave or long-wave UV light.

Color

The violet color of amethyst ranges from a very light, slightly pinkish to a very dark violet.

The color distribution within the crystal is usually irregular. Crystal sectors parallel to the main rhombohedron {10-11} are usually more intensely colored. There, the violet color is concentrated in thin, dark-violet bands parallel to the {10-11} and, less frequently, the {01-11} faces.

In these sectors, amethysts exhibit irregular, fine-lamellar twinning according to the Brazilian law, and the intensely colored planes mark these twin planes.

The color is determined by lattice defects, with an Fe 4+ ion in tetrahedral coordination, i.e., surrounded by four oxygen ions, so-called [FeO 4 ] centers. It is still controversial whether these defects occur at the silicon site (Fe 4+ replacing Si 4+) or at interstitial sites (Fe 4+ in a normally empty tetrahedral gap in one of the six-sided channels of the quartz structure).

These defects arise from defects involving Fe 3+ in a tetrahedral void caused by irradiation (e.g., from radioactive decay). This radiation removes an electron from the Fe ion, thus ionizing Fe 3+ to Fe 4+. The radiation dose required to produce the amethyst color, for example, in granitic rocks, is generated solely by the decay of the naturally occurring 40 K isotope over approximately six million years.

Amethysts can also be produced from colorless iron-containing quartz by irradiation with gamma radiation.

Under the influence of UV rays (e.g. from sunlight), amethysts lose their color quite quickly.

Amethysts also lose their color when heated. They usually develop a citrine coloration, caused by submicroscopic iron oxide mixtures, and more rarely a green prasiolite coloration or become completely colorless.

Characteristics

Although it is very resistant to acids, amethyst is very susceptible to heat. In fact, when heated to over 300°C, its color changes to brown, yellow, orange, or green, depending on its quality and place of origin:

450 °C: turns yellow

500 °C: takes on a strong orange color (burnt amethyst)

600 °C: becomes very milky

These color changes are due to changes in the valence of the iron it contains, among other factors. Amethyst's original color can be restored by irradiating it. It differs from other heat-treated stones in that it exhibits bluish-purple and reddish-purple dichroism.

It lacks a characteristic light absorption spectrum. It often exhibits inclusions in the form of parallel markings, known as zebra stripes and tiger scratches, caused by rhombohedral twinning.

History

Amethyst was used as a gemstone by the ancient Egyptians and was largely employed in antiquity for intaglio engraved gems.

The ancient Greeks believed amethyst gems could prevent intoxication, while medieval European soldiers wore amethyst amulets as protection in battle in the belief that amethysts heal people and keep them cool-headed.(p 77) Beads of amethyst were found in Anglo-Saxon graves in England. Anglican bishops wear an episcopal ring often set with an amethyst, an allusion to the description of the Apostles as "not drunk" at Pentecost in Acts 2:15.

A large geode, or "amethyst-grotto", from near Santa Cruz in southern Brazil was presented at a 1902 exhibition in Düsseldorf, Germany.

Cultural history

Ancient Greece

The Greek word amethystos may be translated as "not drunken", from Greek a-, "not" + methustos, "intoxicated". Amethyst was considered to be a strong antidote against drunkenness.

In his poem "L'Amethyste, ou les Amours de Bacchus et d'Amethyste" (Amethyst or the loves of Bacchus and Amethyste), the French poet Rémy Belleau (1528–1577) invented a myth in which Bacchus, the god of intoxication, of wine, and grapes was pursuing a maiden named Amethyste, who refused his affections. Amethyste prayed to the gods to remain chaste, a prayer which the chaste goddess Diana answered, transforming her into a white stone. Humbled by Amethyste's desire to remain chaste, Bacchus poured wine over the stone as an offering, dyeing the crystals purple.(p 58–59)

Variations of the story include that Dionysus had been insulted by a mortal and swore to slay the next mortal who crossed his path, creating fierce tigers to carry out his wrath. The mortal turned out to be a beautiful young woman, Amethystos, who was on her way to pay tribute to Artemis. Her life was spared by Artemis, who transformed the maiden into a statue of pure crystalline quartz to protect her from the brutal claws. Dionysus wept tears of wine in remorse for his action at the sight of the beautiful statue. The god's tears then stained the quartz purple.

This myth and its variations are not found in classical sources. However, the goddess Rhea does present Dionysus with an amethyst stone to preserve the wine-drinker's sanity in historical text.

Chinese Tibetans consider amethyst sacred to the Buddha and make prayer beads from it. Amethyst is considered the birthstone of February. In the Middle Ages, it was considered a symbol of royalty and used to decorate English regalia. In the Old World, amethyst was considered one of the cardinal gems, in that it was one of the five gemstones considered precious above all others, until large deposits were found in Brazil.

Value

Until the 18th century, amethyst was included in the cardinal, or most valuable, gemstones (along with diamond, sapphire, ruby, and emerald), but since the discovery of extensive deposits in locations such as Brazil, it has lost most of its value. It is now considered a semiprecious stone.

Collectors look for depth of color, possibly with red flashes if cut conventionally. As amethyst is readily available in large structures, the value of the gem is not primarily defined by carat weight. This is different from most gemstones, since the carat weight typically exponentially increases the value of the stone. The biggest factor in the value of amethyst is the color displayed.

The highest-grade amethyst (called deep Russian) is exceptionally rare. When one is found, its value is dependent on the demand of collectors; however, the highest-grade sapphires or rubies are still orders of magnitude more expensive than amethyst.

Geographic distribution

Amethyst is found in many locations around the world. Between 2000 and 2010, the greatest production was from Marabá and Pau d'Arco, Pará, and the Paraná Basin, Rio Grande do Sul, Brazil; Sandoval, Santa Cruz, Bolivia; Artigas, Uruguay; Kalomo, Zambia; and Thunder Bay, Ontario. Lesser amounts are found in many other locations in Africa, Brazil, Spain, Argentina, Russia, Afghanistan, South Korea, Mexico, and the United States.

Deposits

Amethyst is relatively common, its presence is known in about 2000 localities, although the deposits where high quality specimens have been obtained, for cutting as a gem or for mineral collecting or decoration are not so common. Among them, the following may be highlighted:

Argentina

Deposits are found in various locations, the main one being Wanda, a town whose economy revolves around this stone and where its quality is among the best in the world. A 3-ton geode has been found here. The origin of the deposit dates back some 150 million years. 

Brazil 

Amethyst is produced in abundance in the state of Rio Grande do Sul in Brazil where it occurs in large geodes within volcanic rocks. Many of the hollow agates of southwestern Brazil and Uruguay contain a crop of amethyst crystals in the interior. Artigas, Uruguay and neighboring Brazilian state Rio Grande do Sul are large world producers, with lesser quantities mined in Minas Gerais and Bahia states.

The best-known deposits are found around the small town of Ametista do Sul, and in Riaí, both in Rio Grande do Sul, Minas Gerais. Amethyst appears as crystals formed mainly by two rhombohedrons, with a poorly developed prism, covering amygdaloid cavities, which can reach metric sizes, in a basalt. In the areas of Marabá and Pau d'Arco, in the state of Pará, there are also important deposits. Other deposits of the same type are found in the Quarai area, on the border with Uruguay. 

Bolivia 

Gem-quality amethyst is also found at the Anahi mine, known as the typical locality for ametrine, and at the Ayoreita mine, both in the La Gaiba mining area of Santa Cruz. 

Canada

Amethyst is relatively common in the Canadian provinces of Ontario and Nova Scotia. The largest amethyst mine in North America is located in Thunder Bay, Ontario. Thunder Bay is located on the northeast coast of Lake Superior, in Ontario. It is an area with a variety of exploited mineralizations, including silver. Amethyst was discovered around 1845, and it is currently the official gem of the province of Ontario. It is found forming druses in which the crystals have a very poorly developed prism. Opaque and red-colored specimens are common due to iron oxide inclusions. 

Egypt

Amethyst deposits in Egypt were important during the Pharaonic period, until the reign of the Ptolemies, when they were exhausted. One is located in Wadi al-Hudi, 35 km southwest of Aswan, and the other in Abu Diyeiba, on the Red Sea coast. 

Spain 

In the mining area of the Sierra Minera de Cartagena-La Unión (Murcia), amethysts were probably extracted for use as gems since Roman times, and their extraction is well documented in the 16th century, especially in the Cabezo de Don Juan area. Until the appearance of Brazilian amethysts, they were highly prized. In modern times, they have only been extracted occasionally as collector's items. 

South Korea

Amethyst is also found and mined in South Korea. The large opencast amethyst vein at Maissau, Lower Austria, was historically important, but is no longer included among significant producers. Much fine amethyst comes from Russia, especially near Mursinka in the Ekaterinburg district, where it occurs in drusy cavities in granitic rocks. Amethyst was historically mined in many localities in south India, though these are no longer significant producers. One of the largest global amethyst producers is Zambia in southern Africa, with an annual production around 1000 tons.

United States

The Four Peaks deposit in Maricopa County, Arizona, has been known for over a century. Amethyst crystals occur in fractures in quartzite and are formed by the combination of two rhombohedrons, with the prism faces poorly developed or absent. The color appears zoned in the crystals. 

Amethyst occurs at many localities in the United States. The most important production is at Four Peaks, Gila and Maricopa Counties, Arizona, and Jackson's Crossroads, Wilkes County, Georgia.

Smaller occurrences have been reported in the Red Feather Lakes, near Fort Collins, Colorado; Amethyst Mountain, Texas; Yellowstone National Park; Delaware County, Pennsylvania; Haywood County, North Carolina; Deer Hill and Stow, Maine, and in the Lake Superior region of Minnesota, Wisconsin, and Michigan.

Amethyst is the official state gemstone of South Carolina. Several South Carolina amethysts are on display at the Smithsonian Museum of Natural History.

Morocco

The most interesting amethyst deposit is the Adrar Tirecht Bou Oudi, in the province of Tata, Souss-Massa region. The amethyst crystals appear isolated within a mudstone, probably originating from the destruction of limestone, in which they were formed by hydrothermal processes. The irregular distribution of color in many specimens is characteristic, with an hourglass shape. 

Mexico

The amethysts of Amatitlán, in the state of Guerrero, are better known for their collector's specimens, with crystals that can reach a length of 30 cm, than for gems intended for carving. The amethyst deposit located near the small town of Piedra Parada, in the municipality of Tatatila, Veracruz, is also very important for its collector's specimens. 

Uruguay

The amethyst deposits of Uruguay are located in the Department of Artigas, on the banks of the Catalán stream, close to the border with Brazil.  Amethyst is the national stone of the Oriental Republic of Uruguay. 

Use as a gemstone

When it is bright and intense in color, amethyst is the most prized quartz variety. The most perfect amethysts are cut for jewelry, and the rest are used to make art objects or, as mineral specimens, for decoration or collecting. Amethyst in gem quality is a popular and widely used gemstone, which is cut for trade either in various faceted shapes (brilliant, navette, briolette) or smooth as a cabochon.

Due to its popularity as a gem, several terms are used in jewelry to describe the different shades of amethyst. Dark purple stones are considered particularly valuable. Other varieties, such as ametrine (a combination of amethyst and citrine in one stone) and the purple-white banded, opaque variety of amethyst quartz, are also valued as gemstones.

"Rose de France" refers to the light lilac amethyst, while "Siberian" refers to the deep violet amethyst with red highlights, the most prized variety. There are also gems that are a natural mixture with areas between amethyst and citrine, which have been given the names ametrine or bolivianite. Finally, heated amethyst, which takes on a yellowish hue, is often marketed as citrine (a naturally rare amber-colored variety of quartz).

Handling and care

The most suitable setting for gem amethyst is a prong or a bezel setting. The channel method must be used with caution.

Amethyst has a good hardness, and handling it with proper care will prevent any damage to the stone. Amethyst is sensitive to strong heat and may lose or change its colour when exposed to prolonged heat or light. Polishing the stone or cleaning it by ultrasonic or steamer must be done with caution.

Manipulations, imitations and syntheses

Firing amethyst (approx. 400°C) produces the yellow to golden color of the citrine variety. Many of the "gold or Madeira topazes " misleadingly sold in the gemstone trade are actually fired amethysts. In some amethysts, firing also produces a greenish hue similar to prasiolite, which is the name under which the fired amethyst is sold. However, the color sometimes disappears completely in the flame, leaving only white quartz.

Fired stones usually develop significantly more cracks and fissures, which are noticeable due to their shine. The crystal tips and surfaces often also burst. As a rule, only low-quality amethysts are fired to "upgrade" them. This is a processing method that changes the color using methods that occur identically in nature, without changing any essential properties. Unfired citrine is usually only light in color and, as a rough stone, always has an elongated shape similar to rock crystal. Citrine does not form geodes and is also considerably rarer than amethyst. Fired precious and semi-precious stones do not have to be declared as fired if a permanent color change is caused. 

A cheap, though not very durable, method of enhancing the color of amethyst is to treat it with colored wax, since, unlike agate, amethyst can only be colored superficially. 

Since the 20th century, syntheses of various gemstones, including amethyst, have been produced. These can be distinguished from natural amethysts using gemmological and mineralogical methods, as they exhibit structural variations compared to the naturally occurring stone. 

For costume jewelry, amethyst is imitated by violet glass or offered in composite form as a doublet.

Synthetic amethyst

Synthetic (laboratory-grown) amethyst is produced by a synthesis method called hydrothermal growth, which grows the crystals inside a high-pressure autoclave.

Synthetic amethyst is made to imitate the best quality amethyst. Its chemical and physical properties are the same as those of natural amethyst, and it cannot be differentiated with absolute certainty without advanced gemmological testing (which is often cost-prohibitive). One test based on "Brazil law twinning" (a form of quartz twinning where right- and left-hand quartz structures are combined in a single crystal) can be used to identify most synthetic amethyst rather easily. Synthesizing twinned amethyst is possible, but this type is not available in large quantities in the market.

Treated amethyst is produced by gamma ray, X-ray, or electron-beam irradiation of clear quartz (rock crystal), which has been first doped with ferric impurities. Exposure to heat partially cancels the irradiation effects and amethyst generally becomes yellow or even green. Much of the citrine, cairngorm, or yellow quartz of jewelry is said to be merely "burnt amethyst".

Esotericism

Hildegard of Bingen (1098–1179) described in her "Book of Stones" the use of amethyst as a healing stone against diseases such as skin blemishes and swellings, but also against insect, spider and snake bites, as well as against lice. 

In esoteric circles, amethyst is also used as a healing stone in gemstone therapy to help combat alcoholism and other addictions. It is also said to have cleansing, inspiring, and insightful properties through its influence on the brow chakra. However, there is no scientific evidence for such effects to date, although occasional healing successes have been recorded, which are attributed to the placebo effect. 

Amethyst geodes are also often used for the so-called “ energetic cleansing” and “charging” of other healing stones.

As a zodiac stone, amethyst is associated with the sign of Pisces, according to other sources with the sign of Sagittarius or Capricorn. For balancing the aforementioned brow chakra, amethyst is also mentioned in connection with Virgo. As a planetary stone, it is associated with the planet Neptune (Uyldert 1983, Raphaell 1987) or Pluto (Richardson/Huett 1989). The poet Theodor Körner assigns amethyst to the month of February in his poem "Die Monatssteine" (The Monthly Stones). Furthermore, according to a 1985 publication, the mineral represents Thursday. 

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