Demantoid

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Demantoid
Demantoid - Mineralogisches Museum Bonn (7301).jpg
Demantoid
General
Category Mineral
Formula
(repeating unit)
Ca3Fe2Si3O12
Identification
Color light to deep green
Crystal system cubic
Mohs scale hardness 6.5–7.0
Luster adamantine
Specific gravity 3.84
Optical properties Single Refractive
Refractive index 1.880–1.889[1]
Pleochroism none
Dispersion .057
Common impurities Cr

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,[2] 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.[3][4]

History[edit]

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. 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.[citation needed]

A significant new find of demantoid and andradite took place in Namibia in 1996 at what is now dubbed the "Green Dragon" mine.[5]

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.[6]

Around 2009, there was a significant discovery of demantoid and andradite garnet in Madagascar.[7]

Appearance[edit]

Demantoid is the green variety of andradite garnet,[8] so demantoids are always primarily green (by definition), but the exact shade ranges from a very strong yellowish green to nearly the color of a fine emerald. Some stones have a brownish cast, which is due to iron. Its dispersion (0.057) is unusually high, and this is often visible as "fire" (rainbow-coloured flashes of light), although in some cases the stone's green body colour can render this effect less noticeable. Their luster is adamantine. Demantoid also has a high refractive index of 1.80 to 1.89.

Demantoids are generally small, with finished stones generally under 1 carat (200 mg) and stones over 2 carats (400 mg) are rare. Stones over 3 carats (600 mg) are very rare.

Stones with more intense green coloration are generally highly valued, but lighter stones of yellowish green display substantially more fire. The choice of stone color or fire can therefore be a matter of personal preference, with some preferring the more yellowish-green stones to the green stones.

Horsetails[edit]

Russian demantoid often contain inclusions of chrysotile,[9][10] which is a type of asbestos. These fibers radiate out from a very small crystal of chromite.[11] These inclusions are feathery golden threads that tend to curve and resemble the tail of a horse, and are therefore referred to as horsetail inclusions. In gemology, the presence of such inclusions is regarded as 'diagnostic' for natural demantoid (i.e. these inclusions are not found in any other green gemstone).[11] Some gemstones are more valuable for their inclusions,[12] and 'horsetails' can be regarded as desirable features in demantoid, as they are taken as an indication of prestigious Russian origin,[9][13] although some demantoids from certain other locations (such as Italy and Iran) may also contain 'horsetails', which are regarded as being characteristic of a serpentinite geographic origin,[14] and, on the other hand, not all Russian demantoids actually contain 'horsetails'.[15] The microstructure of some demantoids is believed to be affected by the presence of 'horsetails' (the 'horsetail' typically originates towards the centre of the nodule, with the fibres branching out and radiating towards the surface), whereas horsetail-free demantoids from other sources frequently display flat crystal faces.[16]

References[edit]

  1. ^ Idar-Oberstein, Demantoid International Colored Gemstone Association, accessed online January 25, 2007
  2. ^ Shipley, Robert M. Dictionary of Gems and Gemology, 5th edition, 1951, Gemological Institute of America, pp. 62–3
  3. ^ Wording in "grading report", Gemological Institute of America, July 2012
  4. ^ http://www.gia.edu/research-resources/gems-gemology/issues/winter2012-contents/WN12AB.pdf
  5. ^ ""Subject Index 1981–2010", Gemological Institute of America
  6. ^ "Demantoid Gallery", Mineralogy Database
  7. ^ 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
  8. ^ C. Ghisoli, and F. Caucia, "A contribution to the study of FTIR spectra of opals" (abstract), Gems & Gemology (Spring 2011), Gemological Institute of America
  9. ^ a b Robert Weldon, "A Horse Tale," Gems & Gemology (Sep. 2000)
  10. ^ http://www.gia.edu/doi/10_5741-GEMS_35_4_192
  11. ^ a b http://www.gia.edu/doi/10_5741-GEMS_32_2_100
  12. ^ "Inclusions & Value", Gemological Institute of America
  13. ^ "Demantoid Garnet", Modern Jeweler, posted Jan. 12, 2011
  14. ^ http://www.gia.edu/doi/10_5741-GEMS_45_4_280
  15. ^ http://www.gia.edu/gia-faq-garnet-horsetail-inclusion
  16. ^ "Gem News, Gems & Gemology, Fall 1997, pp. 222–23

External links[edit]