Demantoid

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Demantoid
Category	Mineral
Chemical formula	Ca₃Fe₂Si₃O₁₂
Identification
Color	light to deep green
Crystal system	cubic
Mohs Scale hardness	6.5
Refractive index	1.880 - 1.889^[1]
Optical Properties	Single Refractive
Dispersion	.057
Pleochroism	none
Specific gravity	3.84

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. It is the most expensive and rare of garnet gemstones, with fine specimens commanding prices of thousands of dollars per carat (thousands per gram). 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^[3] that some Russian Demantoid garnets were being routinely subjected to heat treatment in order to enhance their colour. Such treatment is believed to be performed at relatively low temperatures and is not thought to be detectable by gemmological testing.

1 History
2 Appearance
3 Horsetails
4 References

[edit] History

While garnets have been known since ancient times, the demantoid variety was not discovered until 1853 in Russia's central Ural Mountains. The find was north, northwest of Ekaterinburg along the Bobrovka River near the village of Elizaverinskoye. A second find is 75 km. south of Ekaterinburg on the Chusovaya and Chrisolitka Rivers near the village of Poldnevaya. 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 1970's and 1980's. A significant new find in Namibia in 1996. 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. Small stones under 1 carat (200 mg) sell for $200 to $2000 retail. This all depends on how much green color the stone has. Most stones are cut round to show their great brillance.

[edit] Appearance

Demantoid by definition is always green, but the exact shade ranges from a very light yellowish peridot green to nearly the color of a fine emerald. Many stones have a brownish cast. Stones with more intense green coloration are more 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 less valuable but more lively yellowish-green stones.

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. Demantoid also has a high refractive index of 1.80 to 1.89.

Demantoids are generally small, with finished stones over one carat (200 mg) uncommon and stones over two carats (400 mg) quite rare. Clean stones over five carats (1 g) are considered world-class.

[edit] Horsetails

Russian demantoid nearly always contains inclusions of byssolite and/or chrysotile^[4], both of which are types of asbestos. 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 gemmology, the presence of such inclusions is regarded as diagnostic for natural demantoid. Unlike most inclusions which reduce the value of a gemstone, aesthetically pleasing horsetail inclusions can substantially increase the value of a stone. Namibian demantoid does not contain horsetails (although at least one contradictory report is known^[5]). It is generally regarded as inferior to Russian demantoid. Even horsetails which are not visible to the naked eye are valuable in identifying the origin of a stone.

The microstructure of some demantoids is believed to be affected by the presence of 'horsetails'. Demantoids containing horsetails tend to form as pebble-like nodules without well-defined crystal faces^[6] (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^[7].