amethyst

Amethyst
Amethyst cluster from Magaliesburg, South Africa.
General
Category	Mineral variety
Chemical formula	Silica (silicon dioxide, SiO2)
Identification
Color	Purple, violet
Crystal habit	6-sided prism ending in 6-sided pyramid (typical)
Crystal system	rhombohedral class 32
Twinning	Dauphine law, Brazil law, and Japan law
Cleavage	None
Fracture	Conchoidal
Mohs scale hardness	7–lower in impure varieties
Luster	Vitreous/glossy
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	None
Melting point	1650±75 °C
Solubility	insoluble in common solvents
Other characteristics	Piezoelectric

Amethyst is a violet variety of quartz often used in jewelry. The name comes from the Ancient Greek ἀ a- ("not") and μέθυστος methustos ("intoxicated"), a reference to the belief that the stone protected its owner from drunkenness. The ancient Greeks and Romans wore amethyst and made drinking vessels of it in the belief that it would prevent intoxication. It is one of several forms of quartz. Amethyst is the traditional birthstone for February.

Structure

Amethyst is a purple variety of quartz (SiO₂) and owes its violet color to irradiation, iron impurities (in some cases in conjunction with transition element impurities), and the presence of trace elements, which result in complex crystal lattice substitutions.^[1][2][3] The hardness of the mineral is the same as quartz, thus it is suitable for use in jewelry.

Hue and tone

Amethyst occurs in primary hues from a light pinkish violet to a deep purple. Amethyst may exhibit one or both secondary hues, red and blue. The ideal grade is called "Deep Siberian" and has a primary purple hue of around 75–80%, with 15–20% blue and (depending on the light source) red secondary hues.^[4] Green quartz is sometimes incorrectly called green amethyst, which is an actual misnomer and not an acceptable name for the material, the proper terminology being Prasiolite. It is actually against FTC Guidelines to call prasiolite green amethyst.^{[citation needed]} Other names for green quartz are vermarine, greened amethyst, or lime citrine.

Faceted amethyst

Of very variable intensity, the color of amethyst is often laid out in stripes parallel to the final faces of the crystal. One aspect in the art of lapidary involves correctly cutting the stone to place the color in a way that makes the tone of the finished gem homogeneous. Often, the fact that sometimes only a thin surface layer of violet color is present in the stone or that the color is not homogeneous makes for a difficult cutting.

The color of amethyst has been demonstrated to result from substitution by irradiation of trivalent iron (Fe³⁺) for silicon in the structure,^[3][5] 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,^[6] 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]

History

Roman intaglio engraved gem of Caracalla in amethyst, once in the Treasury of Sainte-Chapelle.

Amethyst was used as a gemstone by the ancient Egyptians and was largely employed in antiquity for intaglio engraved gems.^[7]

The Greeks believed amethyst gems could prevent intoxication,^[8] while medieval European soldiers wore amethyst amulets as protection in battle – the reason for this being that amethysts are believed to heal people and keep them cool-headed.^[9] Beads of amethyst were found in Anglo-Saxon graves in England.^[10]

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

In the 19th century, the color of amethyst was attributed to the presence of manganese. However, since it is capable of being greatly altered and even discharged by heat, the color was believed by some authorities to be from an organic source. Ferric thiocyanate has been suggested, and sulfur was said to have been detected in the mineral.^[11]

Synthetic amethyst

Synthetic amethyst is produced by gamma-ray, x-ray or electron beam irradiation of clear quartz which has been first doped with ferric impurities. On exposure to heat, the irradiation effects can be partially cancelled and amethyst generally becomes yellow or even green, and much of the citrine, cairngorm, or yellow quartz of jewelry is said to be merely "burnt amethyst".^[12]

Synthetic amethyst is made to imitate the best quality amethyst. Its chemical and physical properties are so similar to that of natural amethyst that it can not be differentiated with absolute certainty without advanced gemnological testing (which is often cost-prohibitive). There is 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^[13]) which can be used to identify synthetic amethyst rather easily. It is possible to synthesize twinned amethyst, but this type is not available in large quantities in the market.^[4]

Mythology

The Greek word "amethystos" may be translated as "not drunken", from Greek a-, "not" + methustos, "intoxicated".^[14] Amethyst was considered to be a strong antidote against drunkenness, which is why wine goblets were often carved from it. In Greek mythology, Dionysus, the god of intoxication, and of wine, was pursuing a maiden named Amethystos, who refused his affections. Amethystos prayed to the gods to remain chaste, a prayer which the goddess Artemis answered, transforming her into a white stone. Humbled by Amethystos's desire to remain chaste, Dionysus poured wine over the stone as an offering, dyeing the crystals purple.

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.^[15] Another variation involves the titan Rhea presenting Dionysus with the amethyst stone to preserve the wine-drinker's sanity.^[16]

Geographic distribution

Amethyst is produced in abundance from the state of Minas Gerais 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 exceeding in quantity Minas Gerais, as well as Mato Grosso, Espirito Santo, Bahia, and Ceará states, all amethyst producers of importance in Brazil.

It is also found and mined in South Korea. The largest opencast amethyst vein in the world is in Maissau, Lower Austria. Much fine amethyst comes from Russia, especially from near Mursinka in the Ekaterinburg district, where it occurs in drusy cavities in granitic rocks. Many localities in south India yield amethyst. One of the largest global amethyst producers is Zambia in southern Africa with an annual production of about 1000 tonnes.

Amethyst occurs at many localities in the United States.^[11] Among these may be mentioned: the Mazatzal Mountain region in Gila and Maricopa Counties, Arizona; Red Feather Lakes, near Ft 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, Michigan, and Ontario in Canada. Amethyst is relatively common in Ontario, and in various locations throughout Nova Scotia. The largest amethyst mine in North America is located in Thunder Bay, Ontario.

An amethyst geode that formed when large crystals grew in open spaces inside the rock.

Value

Up until the 18th century, amethyst was included in the cardinal, or most valuable, gemstones (along with diamond, sapphire, ruby, and emerald). However, since the discovery of extensive deposits in locations such as Brazil, it has lost most of its value.

Collectors look for depth of color, possibly with red flashes if cut conventionally.^[17] The highest grade amethyst (called "Deep Russian") is exceptionally rare and therefore, when one is found, its value is dependent on the demand of collectors. It is, however, still orders of magnitude lower than the highest grade sapphires or rubies (padparadscha sapphire or "pigeon's blood" ruby).^[4]

See also

• List of minerals
• Specimen Ridge

References

Attribution

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Amethyst". Encyclopædia Britannica (11th ed.). Cambridge University Press. 

External links

Wikimedia Commons has media related to: Amethyst

• Mindat: mineralogical information, crystallography and nomenclature
• gemstone.org - International colored Gemstone Association