| Amethyst | |
| Category: | Silicate mineral |
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| Formula: | Silica (silicon dioxide, SiO) |
| System: | Trigonal |
| Class: | Trapezohedral (32) |
| Symmetry: | P321 (no. 154) |
| Color: | Purple, violet, dark purple |
| Habit: | 6 sided prism ending in 6 sided pyramid (typical) |
| Twinning: | Dauphine law, Brazil law, and Japan law |
| Cleavage: | None |
| Fracture: | Conchoidal |
| Mohs: | 7 (lower in impure varieties) |
| Luster: | Vitreous/glassy |
| Refractive: | nω = 1.543–1.553 = 1.552–1.554 |
| Opticalprop: | Uniaxial (+) |
| Birefringence: | +0.009 (B-G interval) |
| Pleochroism: | Weak to moderate purple/reddish purple |
| Streak: | White |
| Gravity: | 2.65 constant; variable in impure varieties |
| Melt: | ±75 °C |
| Solubility: | Insoluble in common solvents |
| Diaphaneity: | Transparent to translucent |
| Other: | Piezoelectric |
Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from a-, "not" and (Ancient Greek) / (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 intoxification.
Amethyst, a semiprecious stone, is often used in jewelry.
Amethyst is a purple variety of quartz and owes its violet color to irradiation, impurities of iron and in some cases other transition metals, and the presence of other trace elements, which result in complex crystal lattice substitutions.[1] [2] [3] The irradiation causes the iron ions that replace Si in the lattice to lose an electron and form a color center.[4] 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.
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.[5] 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.[6] "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.[7]
Green quartz is sometimes called green amethyst; the scientific name is prasiolite.[8] 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.[9] [10]
The color of amethyst has been demonstrated to result from substitution by irradiation of trivalent iron (Fe3+) for silicon in the structure,[3] [11] in the presence of trace elements of large ionic radius,[2] 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,[3] but when heated, turns yellow-orange, yellow-brown, or dark brownish and may resemble citrine,[12] 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.[3] It does not fluoresce under either short-wave or long-wave UV light.
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.[13] Amethyst is produced in abundance in the state of Rio Grande do Sul in Brazil where it occurs in large geodes within volcanic rocks.[14] [15] [16] [17] 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.[13] The largest amethyst geode found as of 2007 was the Empress of Uruguay, found in Artigas, Uruguay in 2007. It stands at a height of 3.27 meters, lies open along its length, and weighs 2.5 tons.
Amethyst is also found and mined in South Korea.[18] The large opencast amethyst vein at Maissau, Lower Austria, was historically important, but is no longer included among significant producers.[13] Much fine amethyst comes from Russia, especially near Mursinka in the Ekaterinburg district, where it occurs in drusy cavities in granitic rocks.[13] Amethyst was historically mined in many localities in south India, though these are no longer significant producers.[13] One of the largest global amethyst producers is Zambia in southern Africa, with an annual production around 1000 tons.[19]
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.[13]
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 relatively common in the Canadian provinces of Ontario and Nova Scotia. The largest amethyst mine in North America is located in Thunder Bay, Ontario.[13]
Amethyst is the official state gemstone of South Carolina. Several South Carolina amethysts are on display at the Smithsonian Museum of Natural History.[20]
Amethyst was used as a gemstone by the ancient Egyptians and was largely employed in antiquity for intaglio engraved gems.[21]
The ancient Greeks believed amethyst gems could prevent intoxication,[22] while medieval European soldiers wore amethyst amulets as protection in battle in the belief that amethysts heal people and keep them cool-headed.[23] Beads of amethyst were found in Anglo-Saxon graves in England.[24] 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.[25]
A large geode, or "amethyst-grotto", from near Santa Cruz in southern Brazil was presented at a 1902 exhibition in Düsseldorf, Germany.
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[26]) 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.[6]
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".[27]
The Greek word amethystos may be translated as "not drunken", from Greek a-, "not" +, "intoxicated". Amethyst was considered to be a strong antidote against drunkenness.