Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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Demantoid Garnet

January, 2000

Demantoid garnet, named for its diamond-like luster and dispersion, is the name for medium to dark green to yellow-green andradite garnet. Their rich green color is contributed by chromium and as a result they show a red reaction when viewed through a Chelsea filter. The finest specimens come from the Ural Mts. in Russia although some can be found in Italy and Zaire. Yellow andradrite is considered a separate variety called topazolite. Although the dispersion is the highest of any garnet (and higher than diamond) often this "fire" is somewhat masked by the rich green body color. Frequently thin, asbestos fibers are present in characteristic formations referred to as "horsetails"; their presence is proof positive of identity in this variety, and raise the value of a piece. Somewhat softer than other garnet species, some care is necessary in setting and wearing. Demantoids were historically important in high end Victorian jewelry but became unavailable for many years due to lack of production at the mine sources. In the US, Tiffany's, in particular, made extensive use of demantoid in the late 19th century. African stones are available, but characteristically lack both the horsetails and the saturated deep green color that is so desirable. Small amounts of fine Russian material have recently come on the market due to increased mining efforts, raising hopes that we once again might see jewelry pieces designed around this spectacular gem.


Value

Good green color is the first determiner of value in demantoids with highly yellowed stones (which have considerable iron in their structure) worth considerably less. Horsetails which are primarily indicative of Russian orgin, add strongly to value when they do not excessively obscure the clarity. As in Benitoite, another highly dispersive but richly colored stone, there is a trade off between richness of body color and visible dispersion. Collectors differ in their opinion as to which is more desirable, both extremes are valuable and have their advocates. Demantoid prices are the highest of any type of garnet and one among the highest for colored gemstones in general.

Gemological Data:

Makeup: Calcium, iron, silicate

Luster: Vitreous to adamantine

Hardness: 6.5

Crystal structure: cubic

Fracture: conchoidal

Cleavage: none

Density: 3.85

RI: 1.85-1.89

Birefringence: none

Pleiochroism: none

Dispersion: .057


Stones Currently Available:

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Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment