Garnet has been a popular gemstone for centuries due to its very good hardness (7 to 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 (spinel and diamond are examples of other singly refractive gemstones).
Garnet has been regarded by some buyers as a rather common gemstone. Yet, 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 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 to 4.1 and a refractive index of 1.88 to 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 zircon and diamond has greater brilliance. Demantoid also has remarkable dispersion, or fire, that exceeds that of diamond. Indeed, the very name demantoid means "diamond-like luster."
Demantoid by definition is always green, and the green hue is caused by traces of chromium. Colors range from a light yellowish-green (similar to peridot) to a deep-green that approaches the color of fine emerald. Specimens that are an intense green are more highly valued, but lighter stones display substantially more fire. Demantoid garnet is typically only found in small sizes. Specimens weighing over one carat are uncommon and stones weighing over 2 carats are very rare.
While most garnets have been known since ancient times, demantoid garnet was not discovered until 1853, in the Ural Mountains of 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 highly valued. Distinctive 'horsetail' inclusions are the telltale sign of Russian demantoid, since to date, these inclusions have not been found in the Namibian material. Russian demantoid nearly always contains inclusions of byssolite and/or chrysotile, both of which are types of asbestos. These inclusions take the form of feathery golden threads that tend to curve and resemble the tail of a horse.
- First Published: July-03-2008
- Last Updated: June-12-2014
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