200px|thumb|granat andradyt

'''Demantoid''' – minerał z gromady krzemianów, zaliczany do grupy granatów (gemmologiczna odmiana andradytu). Minerał bardzo rzadki.

Nazwa pochodzi od niderlandzkiego ''demant'' - oszlifowanego do diamentu.<br>

===Właściwości===
*Wzór chemiczny - '''Ca<sub>3</sub>Fe<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>''' - krzemian wapnia i żelaza
*Układ krystalograficzny: regularny
*Barwa - trawiastozielona do szmaragdowozielonej wywołana obecnością chromu.
*Łupliwość - niewyraźna, równoległa do ścian postaci
*Przełam – nierówny, muszlowy
*Rysa - biała
*Twardość w skali Mohsa - 6.5 -7
*Gęstość - 3.85 g/cm<sup>3</sup>
*Połysk - szklisty lub tłusty
*Dyspersja - 0.057

Tworzy kryształy o postaci dwunastościanu regularnego. Występuje w formie skupień  zbitych, ziarnistych oraz naskorupień. Jest kruchy, przezroczysty. Niektóre okazy odbijają światło silniej niż diamenty. 

== Występowanie: ==
Występuje zazwyczaj w skałach metamorficznych i w ultrazasadowych skałach magmowych. Stanowi składnik wielu osadów aluwialnych (piasków i żwirów).   

'''Miejsca występowania:'''  Został po raz pierwszy znaleziony przez poszukiwaczy złota w pobliżu Niżnego Tagilska na Włoszech. Także: USA, Niemcy, Słowacja.

{{Infobox mineral
| name        = Demantoid
| category    = 
| boxwidth    = 
| boxbgcolor  = 
| image       = 
| imagesize   = 
| caption     = 
| 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
| luster      = 
| polish      = 
| refractive  = 1.880 - 1.889<ref name="ICA">Idar-Oberstein, [http://www.gemstone.org/gem-by-gem/english/demantoid.html Demantoid] International Colored Gemstone Association, accessed online January 25, 2007</ref>
| opticalprop = Single Refractive
| birefringence = 
| dispersion  = .057
| pleochroism = none
| fluorescence= 
| absorption  =
| streak      = 
| gravity     = 3.84
| density     = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = 
| other       = 
}}
'''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 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 misnomers; ''olivine'',<ref>Shipley, Robert M. ''Dictionary of Gems and Gemology'', 5th edition, 1951, Gemological Institute of America, pp62-3</ref> and ''Uralian emerald''.

In approximately 2003, reports began to circulate in the trade<ref>[http://www.palagems.com/gem_spectrum7.htm The Gem Spectrum: Russian Demantoid Garnet [Vol. 7, Feb. 2003&#93;<!-- Bot generated title -->]</ref> 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.

==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 took place in Namibia in 1996 at what is now dubbed the "Green Dragon" mine. 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.

==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.