|
Demantoid is the green gemstone variety of the mineral andradite, a member of the garnet group of minerals. Andradite is a calcium- and iron-rich garnet. The chemical formula is Ca3Fe2(SiO4)3 with chromium substitution as the cause of the demantoid green color. Ferric iron is the cause of the yellow in the stone. It has the misnomers ''olivine'',〔Shipley, Robert M. ''Dictionary of Gems and Gemology'', 5th edition, 1951, Gemological Institute of America, pp. 62–3〕 and ''Uralian emerald''. In approximately 2003, reports began to circulate in the trade that some Russian demantoid garnets were being routinely subjected to heat treatment in order to enhance their color. Such treatment is believed to be performed at relatively low temperatures and is thought not to be detectable by gemological testing.〔Wording in "(grading report )", Gemological Institute of America, July 2012〕〔http://www.gia.edu/research-resources/gems-gemology/issues/winter2012-contents/WN12AB.pdf〕 ==History== While garnets have been known since ancient times, the demantoid variety was not discovered until 1868 in Russia's western central Ural Mountains. The find was about 110 kilometers north, northwest of Ekaterinburg along the Bobrovka River near the village of Elizavetinskoye. This is an alluvial deposit. Miners were immediately stunned by the highly refractive nature of the gem material, which is atypical for garnet. They began comparing it to diamond and referred to it as "Demantoid" which comes from the French, demant, meaning diamond. The reason is obvious, with its high brilliance and dispersion〔http://www.gemsociety.org/article/demantoid/〕 A second find is 75 km. south of Ekaterinburg on the Chusovaya and Chrisolitka Rivers southwest of the village of Poldnevaya. Deposits are also found underground up to 3 meters deep in the demantoid placier. There are five deposits of demantoid in this area. Possessing an unusual green color and a dispersion greater than that of diamond, it quickly became a treasured and expensive gemstone. From the time of the demantoids find until about 1919, they were popular in Russia as the famous Peter Carl Fabergé made jewelry with them. With communist Russia, these gems went out of style. More stones were then found in the Bobrovka River in the 1970s and 1980s. Around 1999 very limited production occurred in the central Ural Mountains. Many of the stones found then are for sale today. Mining takes place along the rivers today, but some mining is still done secretively. A significant new find of demantoid and andradite took place in Namibia in 1996 at what is now dubbed the "Green Dragon" mine.〔"("Subject Index 1981–2010 )", Gemological Institute of America〕 In addition to the commercially-important deposits in Russia and Namibia, demantoids are also found in other locations including Italy (Val Malenco, Lombardy), Iran (Kerman) and Afghanistan.〔"(Demantoid Gallery )", Mineralogy Database〕 Around 2009, there was a significant discovery of demantoid and andradite garnet in Madagascar.〔Federico Pezzotta, Ilaria Adamo, and Valeria Diella. "(Demantoid and Topazolite from Antetezambato, Northern Madagascar: Review and New Data )," ''Gems & Gemology'' (Spring 2011), Gemological Institute of America〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Demantoid」の詳細全文を読む スポンサード リンク
|