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Demantoid is a gem variety of the andradite mineral, a specimen of the garnet group. This is a very rare and most expensive variety of garnet.
The color of this garnet is green, often with yellowish or brownish hues, rarely with a blue tint. The most valuable shade is vivid green without any hint of yellowness. Green color comes from the content of chromium and iron, titanium gives yellow shades.
This gem was at first time found in the middle of the 19th century in Russia, in the Urals Mountains. Now mining is conducted in other countries, but the Ural samples are valued more than others. The price of demantoids from the Urals is several times greater than from Namibia or Madagascar. Natural uralian demantoid is a kind of established brand, just like a ruby from Burma or an emerald from Colombia. A characteristic feature of the Ural crystals is the presence of beautiful fibrous inclusions of the bissolite in the form of a “horsetail”. The presence of horsetails can even increase the price. Good quality demantoids are in great demand in the global market. Japanese collectors very like demantoid from the Urals. It is a pity that Ural specimens in faceted form rarely exceed 1-2 carats in mass. The properties of the green garnet stone from Africa are slightly different from the stones from the Urals. African demantoids are often larger, but they are usually inferior to the Urals in beauty, their color is rarely bright green.
The most valuable classic demantoids are mined in Russia, in the Urals. Three main Ural deposits can be distinguished: Karkodinskoye, Poldnevskoye and placers of the Bobrovskoe deposit. Green andradite was also found in the 1940s in Armenia and Azerbaijan, in the 1970s in Chukotka, Kamchatka and Sakhalin, in the 1980s in the Polar Urals. But most of the samples found there cannot be attributed to gem quality. In addition to the Urals, gem-grade demantoids enter the world market mainly from only two sources: from Namibia (since the mid-1990s) and Madagascar (since 2008). Demantoids are also found in small amounts in Iran, Pakistan, and Italy. It should be noted that horsetail inclusions that were previously considered characteristic only for the Ural demantoids were also found, for example, in demantoids from Pakistan, Iran, and Italy. Therefore, to classify a demantoid as a “branded” Ural one, some gemological laboratories pay attention not only to inclusions, but also to chemical content. By the way, demantoids from Namibia and Madagascar, which occupy a significant part of the market, do not contain horsetail inclusions.
Properties
Mineral:
garnet, andradite variety
Chemical formula:
Ca3Fe2 (SiO4) 3
Crystal Systems:
cubic
Mohs scale hardness:
6.5-7
Optical character:
Isotropic
Cleavage:
very imperfect
Density:
3.84 (± 0.03) g / cm3
Luster:
Vitreous to adamantine
Refractive index:
1,888 (+0,007, -0,033)
Birefringence:
None
Dispersion:
0.057
Color:
yellow green, green
Interesting facts
History
The first demantoid was found in the middle of the 19th century in Russia. It was accidentally discovered by children on the Bobrovka river in the Urals. The find was then mistaken for an unusual peridot. The "Ural peridot» was simultaneously mined by prospectors engaged in the extraction of gold and platinum in the Bobrovka valley. When the finnish mineralogist Nils Nordenskiöld arrived in Nizhny Tagil in 1849 he was shown the samples found. Nordenskiöld's studies showed that the mineral is not peridot, but a chromium-containing variety of andradite garnet with an unusual color from peridot to emerald. In 1854 he spoke about the results of his research at a meeting of the Imperial Mineralogical Society, and proposed the name “demantoid” for this stone in honor of its strong diamond-like “fire”. The World Exhibition held in Paris in 1889, contributed to the growth of the mineral's popularity. After it, the gem finally conquered both amateurs and professionals. Among gem lovers were Emperor Nicholas II and the English king Edward VII.
Enhancement
Gem varieties of garnet are usually not subjected to any kind of treatment. But this stone can sometimes be subjected to low-temperature heat treatment to remove the brownish tint and “open” a greener color, or to slightly lighten the stone. The elimination of a brownish tint in demantoids during heat treatment is associated with a change in the valence of iron. Heat treatment is common for Ural specimens, but there is no exact information on stones from Namibia and Madagascar. Demantoids of the Poldnevskoye deposit can be either heated or unheated. The stone from Bobrovka (Nizhny Tagil) is always without heat treatment, their color does not contain a brown shade. Instances of the Karkodinsky field are always heat-treated; for them, this is a standard and necessary procedure. The fact of heat treatment in expert opinions is indicated by few gemological laboratories in the world. The fact is that this stone can be heated in order to improve color, many foreign market participants began to learn only very recently.
Photo
On the photo: a sample of pear demantoid of Ural origin
On the photo: crystals of green andradite garnet (Madagascar)
On the picture: a faceted round demantoid from Namibia
Rough material from Iran
Vivid green trillion demantoid
Video
Video of demantoid trough the microscope
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