Dc Amethyst
Marva Richardt
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.[1] Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxification.
Amethyst is a purple variety of quartz (SiO
2) 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.[2][3][4]The irradiation causes the iron Fe3+
ions that replace Si in the lattice to lose an electron and form a [FeO
4]0 color center.[5]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.
The color of amethyst has been demonstrated to result from substitution by irradiation of trivalent iron (Fe3+) for silicon in the structure,[4][12] in the presence of trace elements of large ionic radius,[3] 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,[4] but when heated, turns yellow-orange, yellow-brown, or dark brownish and may resemble citrine,[13] but loses its dichroism, unlike genuine citrine. When partially heated, amethyst can result in ametrine.
Amethyst is found in many locations around the world. Between 2000 and 2010, the greatest production was from Marab and Pau d'Arco, Par, and the Paran 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.[14]
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.[14]
Amethyst is also found and mined in South Korea.[19] The large opencast amethyst vein at Maissau, Lower Austria, was historically important,[1] but is no longer included among significant producers.[14] Much fine amethyst comes from Russia, especially near Mursinka in the Ekaterinburg district, where it occurs in drusy cavities in granitic rocks.[1][14] Amethyst was historically mined in many localities in south India,[1] though these are no longer significant producers.[14] One of the largest global amethyst producers is Zambia in southern Africa, with an annual production around 1000 tons.[20]
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.[1]
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[27]) 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.[7]
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".[1][28]
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.[34]
Tibetans consider amethyst sacred to the Buddha and make prayer beads from it.[36][full citation needed] Amethyst is considered the birthstone of February.[37]In the Middle Ages, it was considered a symbol of royalty and used to decorate English regalia.[37] 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.[38]
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.[38] It is now considered a semiprecious stone.[39]
Collectors look for depth of color, possibly with red flashes if cut conventionally.[40]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.[41]
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.[7]
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.[42]
Amethyst usually is a transparent variety of quartz that comes in shades of purple. Some translucent to opaque amethyst is also found and the purplish zones alternate with white or grayish areas. Color zonation can be very intense in transparent, translucent and opaque varieties. Amethyst is the traditional birth stone for the month of February.
Most of the amethyst that is used in the gem trade now comes from Brazil. Historically, the Ural Mountains in Russia produced many fine amethysts and thousands were acquired by Catherine the Great. The most desired hues ranged from deep purplish-red to purple-red and these stones were called Uralian or Siberian Amethyst, terms that are now color grades rather than source areas.
Amethysts from Brazil are mostly extracted from large geodes that are found in basalts of either Triassic or Jurassic age that crop out in the State of Rio Grande do Sul, in the southeastern corner of the country. Many of these geodes were commonly seen for sale in gem shows in the late 1980's and early 1990's, but fewer of them have been seen in the late 1990's. Many of the geodes with transparent amethyst were broken down to small crystals for the gem trade and this kind of geode is now fairly scarce intact.
Some very attractive amethyst has been produced in Mexico but most of it that has been of desirable gem shades formed in crystals that were quite small and many of the larger crystals were not sufficiently toned to produce fine faceted gems. The amethyst from Mexico has produced many fine, large cabinet specimens that appear in both private and museum collections. When I first took up lapidary in 1961, there were some amethyst tips from Mexico that were nicely colored and produced fine cabochons but they were all nearly opaque. Some of the Mexico specimens had nice small phantom crystals of amethyst in a colorless body of rock crystal.
In North America a great deal of amethyst is extracted from mines in the Thunder Bay, Ontario, Canada, area. There are several commercial mines and several fee localities where one can pay to collect. The status of the mines and fee localities varies from year to year so it is best for one to write or call ahead before planning trips to these areas. Most of the amethyst from Thunder Bay is strongly color zoned and these zonations appear as chevrons if the cut is diagonal to the c-axis of the crystal or sometimes as six triangles of alternating purple and colorless or white areas if the cut is made perpendicular to the c-axis of the crystal. The latter have been called Port Arthur Amethyst. Port Arthur was the name of a port in the Kwantung Province of Manchuria; this area was an historic producer of very fine amethyst. Port Arthur was also the former name of Thunder Bay, Ontario. Which of the Port Arthurs yielded its name to that color zonation of amethyst is currently unknown to me. Some very fine examples of color zoned amethyst similar to the Port Arthur Amethyst appeared in gem shows in the early 1980's; this material was from India.
The strong color zonations of the Port Arthur Amethyst is probably produced by numerous twinned crystals and twinning planes within the crystal. When observing a section of the India material under polarized light and with a condensing lens, very strong interference patterns are readily observed and the normally fine interference figure seen in most quartz appears to be a garbled set of figure superimposed over figure.
c80f0f1006