View source for Demantoid

{{Short description|Green gemstone variety of the mineral andradite}}
{{distinguish|Demonoid|Diamondoid}}
{{Infobox mineral
| name        = Demantoid
| category    = 
| boxwidth    = 
| boxbgcolor  = 
| image       = Andradite-Stilbite-Ca-dem05a.jpg
| imagesize   = 260px
| caption     = Demantoid with [[stilbite]]
| formula     = Ca<sub>3</sub>Fe<sub>2</sub>Si<sub>3</sub>O<sub>12</sub>
| molweight   = 
| color       = light to deep green
| habit       = 
| system      = cubic
| twinning    = 
| cleavage    = 
| fracture    = 
| mohs        = 6.5–7.0
| luster      = adamantine
| polish      = 
| refractive  = 1.880–1.889<ref name="ICA">{{cite web|author=Idar-Oberstein|url=http://www.gemstone.org/gem-by-gem/english/demantoid.html|title=Demantoid|publisher=International Colored Gemstone Association|accessdate=January 25, 2007|url-status=dead|archiveurl=https://web.archive.org/web/20080624171321/http://www.gemstone.org/gem-by-gem/english/demantoid.html|archivedate=June 24, 2008}}</ref>
| opticalprop = Single Refractive
| birefringence = 
| dispersion  = 0.057
| pleochroism = none
| fluorescence= 
| absorption  =
| streak      = 
| gravity     = 3.84
| density     = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = 
| impurities  = Cr
| other       = 
}}
[[File:Granat demantoid - Antetezambato, Ambanja District, Diana (Northern) Region, Antsiranana Province, Madagaskar.jpg|thumb|Garnet demantoid - Antetezambato, Ambanja District, Diana (Northern) Region, Antsiranana Province, Madagaskar.]]
'''Demantoid''' is the green [[gemstone]] variety of the mineral [[andradite]], a member of the [[garnet]] group of [[mineral]]s. Andradite is a [[calcium]]- and [[iron]]-rich garnet. The chemical formula is Ca<sub>3</sub>Fe<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub> 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''<ref>{{cite encyclopedia|author=Shipley, Robert M.|encyclopedia=Dictionary of Gems and Gemology|edition=5th|year=1951|publisher=[[Gemological Institute of America]]|pages=62–63|title=Olivine}}</ref> 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 [[gemology|gemological]] testing.<ref>Wording in {{cite web|url=http://www.atggems.com/certs/cert_demantoid_202pt_cush_GIA.pdf|title=grading report|publisher=Gemological Institute of America|date=July 2012}}</ref><ref>http://www.gia.edu/research-resources/gems-gemology/issues/winter2012-contents/WN12AB.pdf{{dead link|date=June 2016}}</ref>

== History ==
Although garnets have been known since ancient times, the demantoid variety was not discovered until 1851 in [[Russia]]'s western central [[Ural Mountains]].<ref>{{Cite web |title=Andradite (var. demantoid) |url=https://naturalhistory.si.edu/explore/collections/geogallery/10002932 |access-date=18 February 2025 |website=Smithsonian National Museum of Natural History}}</ref>  The find was an alluvial deposit about {{convert|110|km|mi|-1|abbr=on}} 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.<ref>{{cite web|title=Demantoid Garnet Value, Price, and Jewelry Information|publisher=International Gem Society|url=http://www.gemsociety.org/article/demantoid/}}</ref>

A second find was made {{convert|75|km|mi|abbr=on}} to the south of Ekaterinburg, on the Chusovaya and Chrisolitka Rivers southwest of the village of Poldnevaya.  Deposits are also found underground up to {{convert|3|m|ft|0|abbr=on}} 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|date=August 2011}}

A significant new find of demantoid and andradite took place in [[Namibia]] in 1996 at what is now dubbed the "Green Dragon" mine.<ref>{{cite web|url=http://www.gia.edu/research-resources/gems-gemology/indexes/30%20Year-Subject-Index.pdf|title=Subject Index 1981–2010|publisher=Gemological Institute of America}}</ref>

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.<ref>"[http://www.mindat.org/gallery.php?min=1258 Demantoid Gallery]", Mineralogy Database</ref>

The Iranian demantoids exhibit a spatial distribution restricted to two distinct locations: Baft, Kerman province (Southeastern Iran), and Takab, West Azerbaijan (Northwestern Iran). Petrographically, the Baft demantoids are associated with serpentinite schist lithologies, whereas the Takab specimens are exclusively hosted within skarn metamorphic rocks.<ref>[https://gem-a.com/wp-content/uploads/2024/01/2.-Ahadnejad_JoG2022_38_4-Vol-38-4.pdf Ahadnejad et al., 2022]</ref>

Around 2009, there was a significant discovery of demantoid and andradite garnet in Madagascar.<ref>Federico Pezzotta, Ilaria Adamo, and Valeria Diella. "[http://www.gia.edu/research-resources/gems-gemology/issues/spring2011-contents/spring-2011-pezzotta.html Demantoid and Topazolite from Antetezambato, Northern Madagascar: Review and New Data]," ''Gems & Gemology'' (Spring 2011), Gemological Institute of America</ref>

==Appearance==
{{unreferenced section|date=August 2011}}
Demantoid is the green variety of andradite garnet,<ref>C. Ghisoli, and F. Caucia, "[http://www.gia.edu/research-resources/gems-gemology/issues/spring2011-contents/SP11AB.pdf A contribution to the study of FTIR spectra of opals]" (abstract), ''Gems & Gemology'' (Spring 2011), Gemological Institute of America</ref> 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 lustre|adamantine]]. Demantoid also has a high [[refractive index]] of 1.80 to 1.89.

Demantoids are generally small, with finished stones generally under {{convert|1|carat|mg}}  and stones over {{convert|2|carat|mg}} are rare.  Stones over {{convert|3|carat|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]],<ref name="HorseTail">{{cite web|author=Robert Weldon|url=http://www.professionaljeweler.com/archives/articles/2000/sep00/0900gg1.html|title=A Horse Tale|work=Gems & Gemology|date=September 2000|access-date=2007-09-20|archive-date=2020-07-29|archive-url=https://web.archive.org/web/20200729041316/http://professionaljeweler.com/archives/articles/2000/sep00/0900gg1.html|url-status=dead}}</ref><ref>{{cite web|url=http://www.gia.edu/gems-gemology/winter-1999-inclusions-demantoid-garnet-russia-krzemnicki|doi=10.5741/GEMS_35_4_192|publisher=Gemological Institute of America|title=Diopside Needles as Inclusions in Demantoid Garnet from Russia: A Raman Microspectrometric Study|year=1999|volume=35|issue=4|first=Michael S. |last=Krzemnicki|journal=Gems & Gemology}}</ref>  which is a type of [[asbestos]]. These fibers radiate out from a very small crystal of chromite.<ref name="gia.edu">{{cite web|url=http://www.gia.edu/gems-gemology/summer-1996-green-andradite-garnet-phillips|doi=10.5741/GEMS_32_2_100|title=Russian Demantoid, Czar of the Garnet Family|publisher=Gemological Institute of America|year=1996|volume=32|issue=2|first1=Wm. Revell |last1=Phillips |first2=Anatoly S. |last2=Talantsev|journal=Gems & Gemology}}</ref> 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).<ref name="gia.edu"/> Some gemstones are more valuable for their inclusions,<ref>{{cite web|url=http://www.gia.edu/howtobuyagemstone/05_Inclusions_Value.swf|title=Inclusions & Value|publisher=Gemological Institute of America|access-date=2012-01-28|archive-url=https://web.archive.org/web/20130211210938/http://www.gia.edu/howtobuyagemstone/05_Inclusions_Value.swf|archive-date=2013-02-11|url-status=dead}}</ref> and 'horsetails' can be regarded as desirable features in demantoid, as they are taken as an indication of prestigious Russian origin,<ref name="HorseTail"/><ref>{{cite web|url=http://www.modernjeweler.com/online/article.jsp?siteSection=1&id=345&pageNum=2|title=Demantoid Garnet|work=Modern Jeweler|date=January 12, 2011|access-date=November 27, 2011|archive-url=https://web.archive.org/web/20111101133805/http://modernjeweler.com/online/article.jsp?siteSection=1&id=345&pageNum=2|archive-date=November 1, 2011|url-status=dead|df=mdy-all}}</ref> 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,<ref>{{cite web|url=http://www.gia.edu/UK-EN/gems-gemology/winter-2009-demantoid-italy-adamo|doi=10.5741/GEMS_45_4_280|title=Demantoid from Val Malenco, Italy: Review and Update|author=Ilaria Adamo|display-authors=etal|year=2009|volume=45|issue=4|journal=Gems & Gemology}}</ref> and, on the other hand, not all Russian demantoids actually contain 'horsetails'.<ref>{{cite web|url=http://www.gia.edu/gia-faq-garnet-horsetail-inclusion|title=Does a horsetail inclusion guarantee Russian origin?|publisher=Gemological Institute of America}}</ref>
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.<ref>{{cite web|url=http://lgdl.gia.edu/pdfs/f97_GN_demantoid.pdf|title=Gem News|work=Gems & Gemology|date=Fall 1997|pp=222–23|archiveurl=https://web.archive.org/web/20081119154352/http://lgdl.gia.edu/pdfs/f97_GN_demantoid.pdf|archivedate=November 19, 2008|url-status=dead}}</ref>

==References==
<references/>

==External links==
{{Commons category}}
*[http://www.mindat.org/min-1258.html Mineralogy Database with location data]
*[https://web.archive.org/web/20060317155409/http://mineral.galleries.com/minerals/gemstone/demantoi/demantoi.htm Mineral galleries]

{{Gemstone}}
{{Jewellery}}

[[Category:Calcium minerals]]
[[Category:Iron(III) minerals]]
[[Category:Garnet gemstones]]