Demantoid Horsetail Inclusion

Garnet has been a popular gemstone for centuries due to its very good hardness (7-7.5 on the Mohs scale) and great brilliance. Garnet also has a distinctive purity of color since it is one of the very few singly refractive gemstones (along with spinel and diamond).

At the same time, garnet has been regarded by some buyers as a rather common gemstone. But while there are some common and inexpensive garnets such as almandine, there are also some rare and expensive ones. The rarer garnets include spessartite, tsavorite and demantoid.

Of these rare garnets, demantoid is by far the rarest and most valuable. In fact demantoid is one of the rarest of all colored gemstones. Particularly fine demantoids can command prices of thousands of dollars per carat.

Demantoid belongs to the andradite variety of garnet, which is a calcium iron silicate with a density of 3.7-4.1 and a refractive index of 1.88-1.94. The high refractive index makes demantoid the most brilliant of all the garnets; and demantoid garnet has a higher refractive index than sapphire and ruby. Only some some zircons and diamond have greater brilliance. Demantoid also has remarkable dispersion or fire that even exceeds that of diamond. Indeed, the very name demantoid means "diamond-like luster."

Demantoid Garnet

Demantoid by definition is always green, with the green hue caused by traces of chromium. Colors range from a light yellowish green (similar to peridot) to a deep green almost the color of fine emerald. Specimens with a more intense green are more highly valued, but the lighter stones display substantially more fire. You will typically find demantoid garnet only in smaller sizes. Specimens over one carat are uncommon and stones over 2 carats count as very rare.

While most garnets have been known since ancient times, demantoid garnet was not discovered until 1853, in the Ural mountains in Russia. The Russian deposits remained the only source for demantoid until a major find in Namibia in 1996. The Russian demantoid is now rarer, and is generally more high valued. Distinctive 'horsetail' inclusions are the telltale sign of Russian demantoid, since these inclusions are not found thus far in the Namibian material. Russian demantoid nearly always contains inclusions of byssolite and/or chrysotile, both of which are types of asbestos. These inclusions are feathery golden threads that tend to curve and resemble the tail of a horse.