Demantoid
Encyclopedia
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 misnomer
s olivine, 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.
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.
In around 2009, there was a significant discovery of demantoid and andradite garnet in Madagascar.
. 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 carats (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.
, 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. 'Horsetails' can be regarded as desirable features in demantoid, as they are taken as an indication of prestigious Russian origin.
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.
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
variety of the mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
andradite
Andradite
Andradite is a species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.Andradite includes three varieties:* Melanite: Black in color, referred to as "titanian andradite"....
, a member of the garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
group of minerals. Andradite is a calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
- and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
-rich garnet. The chemical formula is Ca3Fe2(SiO4)3 with chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
substitution as the cause of the demantoid green color. Ferric iron is the cause of the yellow in the stone.
It has the misnomer
Misnomer
A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derive their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known.- Sources of misnomers...
s olivine, 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.
History
While garnets have been known since ancient times, the demantoid variety was not discovered until 1853 in Russia'sRussia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
western central Ural Mountains
Ural Mountains
The Ural Mountains , or simply the Urals, are a mountain range that runs approximately from north to south through western Russia, from the coast of the Arctic Ocean to the Ural River and northwestern Kazakhstan. Their eastern side is usually considered the natural boundary between Europe and Asia...
. 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
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
, 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
Peter Carl Fabergé
Peter Karl Fabergé also known as Karl Gustavovich Fabergé in Russia was a Russian jeweller of Baltic German-Danish and French origin, best known for the famous Fabergé eggs, made in the style of genuine Easter eggs, but using precious metals and gemstones rather than more mundane materials.-Early...
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
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
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.
In around 2009, there was a significant discovery of demantoid and andradite garnet in Madagascar.
Appearance
Demantoid by definition is always green, but the exact shade ranges from a very strong yellowish green to nearly the color of a fine emeraldEmerald
Emerald is a variety of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the 10 point Mohs scale of mineral hardness...
. 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
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of 1.80 to 1.89.
Demantoids are generally small, with finished stones generally under 1 carats (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.
Horsetails
Russian demantoid oftens contain inclusions of chrysotileChrysotile
Chrysotile or white asbestos is the most commonly encountered form of asbestos, accounting for approximately 95% of the asbestos in place in the United States and a similar proportion in other countries. It is a soft, fibrous silicate mineral in the serpentine group of phyllosilicates; as such, it...
, which is a type of asbestos
Asbestos
Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
. 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
Gemology
Gemology or gemmology is the science dealing with natural and artificial gems and gemstones. It is considered a geoscience and a branch of mineralogy...
, the presence of such inclusions is regarded as an indication for natural demantoid. 'Horsetails' can be regarded as desirable features in demantoid, as they are taken as an indication of prestigious Russian origin.
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.