Demantoid

Last updated
Demantoid
Andradite-Stilbite-Ca-dem05a.jpg
Demantoid with stilbite
General
Category Mineral
Formula
(repeating unit)		Ca3Fe2Si3O12
Crystal system cubic
Identification
Colorlight to deep green
Mohs scale hardness6.5–7.0
Luster adamantine
Specific gravity 3.84
Optical propertiesSingle Refractive
Refractive index 1.880–1.889 [1]
Pleochroism none
Dispersion 0.057
Common impuritiesCr

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.

Contents

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

Although 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 an alluvial deposit about 110 km (70 mi) from Ekaterinburg, north by northwest along the Bobrovka River, near the village of Elizavetinskoye. Miners were immediately stunned by the highly refractive nature of the gem material, which is atypical for garnet. They began comparing it to diamond and referred to it as "demantoid", from the old German Demant, meaning diamond. The reason is obvious, with its high brilliance and dispersion. [5]

A second find was made 75 km (47 mi) to the south of Ekaterinburg, on the Chusovaya and Chrisolitka Rivers southwest of the village of Poldnevaya. Deposits are also found underground up to 3 m (10 ft) deep in the demantoid placier. In all, 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. In austere Communist Russia, these gems went out of style.

More stones were 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. [6]

In addition to the commercially-important deposits in Russia and Namibia, demantoids are also found in some other places, including Italy (Val Malenco, Lombardy), Iran (Kerman), and Afghanistan. [7]

Around 2009, there was a significant discovery of demantoid and andradite garnet in Madagascar. [8]

Appearance

Demantoid is the green variety of andradite garnet, [9] 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

Russian demantoid often contain inclusions of chrysotile, [10] [11] which is a type of asbestos. These fibers radiate out from a very small crystal of chromite. [12] 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). [12] Some gemstones are more valuable for their inclusions, [13] and 'horsetails' can be regarded as desirable features in demantoid, as they are taken as an indication of prestigious Russian origin, [10] [14] 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, [15] and, on the other hand, not all Russian demantoids actually contain 'horsetails'. [16] 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. [17]

Related Research Articles

<span class="mw-page-title-main">Amethyst</span> Mineral, quartz variety

Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from α- a-, "not" and μεθύσκω methysko / μεθώ metho, "intoxicate", a reference to the belief that the stone protected its owner from drunkenness. Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.

<span class="mw-page-title-main">Emerald</span> Green gemstone, a beryl variety

Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium. Beryl has a hardness of 7.5–8 on the Mohs scale. Most emeralds have lots of material trapped inside during the gem's formation, so their toughness (resistance to breakage) is classified as generally poor. Emerald is a cyclosilicate.

<span class="mw-page-title-main">Gemstone</span> Piece of mineral crystal used to make jewelry

A gemstone is a piece of mineral crystal which, when cut or polished, is used to make jewelry or other adornments. However, certain rocks and occasionally organic materials that are not minerals are also used for jewelry and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity and notoriety are other characteristics that lend value to gemstones.

<span class="mw-page-title-main">Sapphire</span> Gem variety of corundum

Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide (α-Al2O3) with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name sapphire is derived via the Latin sapphirus from the Greek sappheiros (σάπφειρος), which referred to lapis lazuli. It is typically blue, but natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. Red corundum stones also occur, but are called rubies rather than sapphires. Pink-colored corundum may be classified either as ruby or sapphire depending on locale. Commonly, natural sapphires are cut and polished into gemstones and worn in jewelry. They also may be created synthetically in laboratories for industrial or decorative purposes in large crystal boules. Because of the remarkable hardness of sapphires – 9 on the Mohs scale (the third hardest mineral, after diamond at 10 and moissanite at 9.5) – sapphires are also used in some non-ornamental applications, such as infrared optical components, high-durability windows, wristwatch crystals and movement bearings, and very thin electronic wafers, which are used as the insulating substrates of special-purpose solid-state electronics such as integrated circuits and GaN-based blue LEDs. Sapphire is the birthstone for September and the gem of the 45th anniversary. A sapphire jubilee occurs after 65 years.

<span class="mw-page-title-main">Garnet</span> Mineral, semi-precious stone

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

<span class="mw-page-title-main">Ruby</span> Variety of corundum, mineral, gemstone

A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum. Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapphires. Ruby is one of the traditional cardinal gems, alongside amethyst, sapphire, emerald, and diamond. The word ruby comes from ruber, Latin for red. The color of a ruby is due to the element chromium.

<span class="mw-page-title-main">Peridot</span> Green gem-quality mineral

Peridot, sometimes called chrysolite, is a yellowish-green transparent variety of olivine. Peridot is one of the few gemstones that occur in only one color.

<span class="mw-page-title-main">Chrysoberyl</span> Mineral or gemstone of beryllium aluminate

The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl2O4. The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones, although they both contain beryllium. Chrysoberyl is the third-hardest frequently encountered natural gemstone and lies at 8.5 on the Mohs scale of mineral hardness, between corundum (9) and topaz (8).

<span class="mw-page-title-main">Gemology</span> Science dealing with natural and artificial gemstone materials

Gemology or gemmology is the science dealing with natural and artificial gemstone materials. It is a geoscience and a branch of mineralogy. Some jewelers are academically trained gemologists and are qualified to identify and evaluate gems.

<span class="mw-page-title-main">Tanzanite</span> Blue to purple variety of the mineral zoisite

Tanzanite is the blue and violet variety of the mineral zoisite, caused by small amounts of vanadium. Tanzanite belongs to the epidote mineral group. Tanzanite is only found in Simanjiro District of Manyara Region in Tanzania, in a very small mining area approximately 7 km (4.3 mi) long and 2 km (1.2 mi) wide near the Mererani Hills.

<span class="mw-page-title-main">Tsavorite</span> Variety of grossular

Tsavorite or tsavolite is a variety of the garnet group species grossular, a calcium-aluminium garnet with the formula Ca3Al2Si3O12. Trace amounts of vanadium or chromium provide the green color.

<span class="mw-page-title-main">Diamond color</span> Color due to impurities or crystal lattice defects in diamond

A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or color. However, in reality almost no gem-sized natural diamonds are absolutely perfect. The color of a diamond may be affected by chemical impurities and/or structural defects in the crystal lattice. Depending on the hue and intensity of a diamond's coloration, a diamond's color can either detract from or enhance its value. For example, most white diamonds are discounted in price when more yellow hue is detectable, while intense pink diamonds or blue diamonds can be dramatically more valuable. Of all colored diamonds, red diamonds are the rarest. The Aurora Pyramid of Hope displays a spectacular array of naturally colored diamonds, including red diamonds.

<span class="mw-page-title-main">Diamond simulant</span> Diamond-like object which is not a diamond

A diamond simulant, diamond imitation or imitation diamond is an object or material with gemological characteristics similar to those of a diamond. Simulants are distinct from synthetic diamonds, which are actual diamonds exhibiting the same material properties as natural diamonds. Enhanced diamonds are also excluded from this definition. A diamond simulant may be artificial, natural, or in some cases a combination thereof. While their material properties depart markedly from those of diamond, simulants have certain desired characteristics—such as dispersion and hardness—which lend themselves to imitation. Trained gemologists with appropriate equipment are able to distinguish natural and synthetic diamonds from all diamond simulants, primarily by visual inspection.

<span class="mw-page-title-main">Gemological Institute of America</span> Research institute in Carlsbad, California

The Gemological Institute of America (GIA) is a nonprofit institute based in Carlsbad, California. It is dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect buyers and sellers of gemstones by setting and maintaining the standards used to evaluate gemstone quality. The institute does so through research, gem identification and diamond grading services and a variety of educational programs. Through its library and subject experts, GIA acts as a resource of gem and jewelry information for the trade, the public and media outlets.

<span class="mw-page-title-main">Spessartine</span> Nesosilicate, manganese aluminium garnet species

Spessartine is a nesosilicate, manganese aluminium garnet species, Mn2+3Al2(SiO4)3. This mineral is sometimes mistakenly referred to as spessartite.

<span class="mw-page-title-main">Andradite</span> Nesosilicate mineral species of garnet

Andradite is a mineral species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.

<span class="mw-page-title-main">Sunstone</span> Brown-orange feldspar

Sunstone is a microcline or oligoclase feldspar, which when viewed from certain directions exhibits a spangled appearance. It has been found in Southern Norway, Sweden, various United States localities and on some beaches along the midcoast of South Australia.

<span class="mw-page-title-main">Diamond (gemstone)</span> Gemstone

Diamond is one of the best-known and most sought-after gemstones. They have been used as decorative items since ancient times.

<span class="mw-page-title-main">Gemstone irradiation</span> Process to enhance a gemstones optical properties

Gemstone irradiation is a process in which a gemstone is artificially irradiated in order to enhance its optical properties. High levels of ionizing radiation can change the atomic structure of the gemstone's crystal lattice, which in turn alters the optical properties within it. As a result, the gem­stone's color may be significantly altered or the visibility of its inclusions may be lessened.

The Lion of Merelani is a tsavorite gemstone from Tanzania that is in the collection of the National Museum of Natural History of the Smithsonian Institution in Washington, DC. The gemstone weighs 116.76 carats, has 177 facets, and is an intense green color. It was donated in the memory of Campbell R. Bridges, the gemologist who discovered tsavorite, who was murdered in 2009. It is the largest known tsavorite gemstone and the largest fine gemstone ever to be cut and polished in the United States.

References

  1. ↑ Idar-Oberstein. "Demantoid". International Colored Gemstone Association. Archived from the original on June 24, 2008. Retrieved January 25, 2007.
  2. ↑ Shipley, Robert M. (1951). "Olivine". Dictionary of Gems and Gemology (5th ed.). Gemological Institute of America. pp. 62–63.
  3. ↑ Wording in "grading report" (PDF). Gemological Institute of America. July 2012.
  4. ↑ http://www.gia.edu/research-resources/gems-gemology/issues/winter2012-contents/WN12AB.pdf%5B%5D
  5. ↑ "Demantoid Garnet Value, Price, and Jewelry Information". International Gem Society.
  6. ↑ "Subject Index 1981–2010" (PDF). Gemological Institute of America.
  7. ↑ "Demantoid Gallery", Mineralogy Database
  8. ↑ 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
  9. ↑ C. Ghisoli, and F. Caucia, "A contribution to the study of FTIR spectra of opals" (abstract), Gems & Gemology (Spring 2011), Gemological Institute of America
  10. 1 2 Robert Weldon (September 2000). "A Horse Tale". Gems & Gemology. Archived from the original on 2020-07-29. Retrieved 2007-09-20.
  11. ↑ Krzemnicki, Michael S. (1999). "Diopside Needles as Inclusions in Demantoid Garnet from Russia: A Raman Microspectrometric Study". Gems & Gemology. Gemological Institute of America. doi:10.5741/GEMS_35_4_192.
  12. 1 2 Phillips, Wm. Revell; Talantsev, Anatoly S. (1996). "Russian Demantoid, Czar of the Garnet Family". Gems & Gemology. Gemological Institute of America. doi:10.5741/GEMS_32_2_100.
  13. ↑ "Inclusions & Value". Gemological Institute of America. Archived from the original on 2013-02-11. Retrieved 2012-01-28.
  14. ↑ "Demantoid Garnet". Modern Jeweler. January 12, 2011. Archived from the original on November 1, 2011. Retrieved November 27, 2011.
  15. ↑ Ilaria Adamo; et al. (2009). "Demantoid from Val Malenco, Italy: Review and Update". Gems & Gemology. doi:10.5741/GEMS_45_4_280.
  16. ↑ "Does a horsetail inclusion guarantee Russian origin?". Gemological Institute of America.
  17. ↑ "Gem News" (PDF). Gems & Gemology. Fall 1997. pp. 222–23. Archived from the original (PDF) on November 19, 2008.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.