| Demantoid | |
|---|---|
Demantoid |
|
| General | |
| Category | Mineral |
| Chemical formula | 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 not thought to be detectable by gemmological testing.
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While garnets have been known since ancient times, the demantoid variety was not discovered until 1853 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, 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 took place in Namibia in 1996 at what is now dubbed the "Green Dragon" mine.
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.[3]
In around 2009, there was a significant discovery of demantoid and andradite garnet in Madagascar.[4]
Demantoid by definition is always green, but the exact shade ranges from a very strong yellowish green to nearly the color of a fine emerald. Many stones have a brownish cast. 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 very rare.
Stones with more intense green coloration are generelly highly valued, but lighter stones 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.
Russian demantoid oftens contain inclusions of chrysotile,[5] which is a type of asbestos. These fibers radiate out from a very small crystal of chromite. 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 an indication for natural demantoid. If 'horsetails' increase the value of a demantoid is highly questionable, as 'horsetails' might be an interessting inclusion from a gemmological point of view, its an inclusion nonetheless, which reduces the 'fire' and the transparency of the stone. 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.[6]