Demantoid is the green gemstone variety of the mineral andradite , a member of the garnet family of minerals. Andradite is a calcium- and iron-rich garnet, and is a distinct mineral species from the aluminum- and iron-rich reddish almandite garnets typically seen in jewelry stores. It is the most expensive and rare of garnet gemstones, with fine specimens commanding prices of thousands of dollars per carat.

History

While garnets have been known since ancient times, the demantoid variety was not discovered until 1868 in Russia's Ural Mountains. Possessing an unusual green color and a dispersion greater than that of diamond, it quickly became a treasured and expensive gemstone. The initial Ural Mountains deposit was quickly mined out, and the gemstone was largely forgotten in the mid-to-late twentieth century. A significant new find in Namibia in 1996 and recent limited production from the Ural Mountains has now made demantoid available in significant quantities for the first time in nearly a hundred years.

Appearance

Demantoid by definition is always green, but the exact shade ranges from a yellowish peridot green to nearly the color of a fine emerald. Lesser stones may 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 can therefore be a matter of personal preference, with some preferring the less valuable but more lively yellowish-green stones.

Its dispersion (0.057) is the highest of all gemstones, and this is visible in demantoid's intense fire. This generally serves to distinguish it from other stones such as peridot, which can have a similar range of colors but does not share demantoid's fire.

Demantoids are generally small, with finished stones over one carat uncommon and stones over two carats quite rare. Clean stones over five carats are considered world-class.

Horsetails

Russian demantoid nearly always contains inclusions of byssolite, a variety of the mineral actinolite. 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. Unlike most inclusions which reduce the value of a gemstone, aesthetically pleasing horsetail inclusions can substantially increase the value of a stone. They also unambiguously identify the stone as natural demantoid. As Namibian demantoid, which does not contain horsetails, is generally regarded as inferior to Russian demantoid, even horsetails which are not visible to the naked eye are valuable in identifying the origin of a stone.