Demantoid, a yellowish-green to green variety of andradite garnet, is so called because of its high dispersion (“fire,” or the ability of a gem to break up white light into spectral colors) and adamantine luster (the reflective property of a gem’s surface). These gems are diamond-like, and the word “demantoid” comes from the Old German diamant, or Dutch demant, meaning diamond.
History. What was once a most popular gem among jewelry designers, manufacturers, and royalty is now almost forgotten. Demantoids were discovered in Russia’s Ural Mountains in the mid 19th century. They found great favor with the royal court and for close to 60 years were seen in every imaginable jeweled object. But came the Bolshevik revolution in 1917, and the mines were shut. Now, almost a century later, independent mining operations are renewing desire for the fiery green gem.
There are some new demantoid localities of minor importance that should be mentioned, since quantities are limited no matter what the origin. Namibia, Arizona, and Mexico are now producing small, gem-quality, yellowish-green demantoid garnets. However, most are of lighter tone and less saturation than the Russian material.
Color. Color is critical to the varietal name. Demantoids may vary only from yellowish- or brownish-green to green (and in some very rare instances, bluish-green). In any other color they’re andradite. Variations of saturation and tone will affect value and identification. If the color is too light in tone, it may be classified only as andradite, similar to the way light-colored emerald eventually becomes simply light-green beryl. While some dealers prefer the dark, rich, emerald-like, pure and saturated green, others prefer a lighter-toned, more slightly yellowish-green demantoid. The lighter, less-saturated colors allow for more dispersion. Determining the “best” color for demantoid may simply be a matter of personal taste.
Qualities. Expect demantoids to be fairly eye-clean. Unlike most gems, however, this one is known for retaining and even increasing its value by having a unique inclusion. Horsetail-like fibers of asbestos byssolite, or possibly serpentine chrysotile, are often oriented such that they can be seen directly through the table of the gem. Seeing horsetail inclusions will positively identify the stone as demantoid, and from Russia.
Values. Size and color are the important value factors. Gems over 3 cts. are rare, but a proper mix of dispersion and saturated green can be a winning combination. It is rumored that some demantoid can be heat treated to remove the yellowish or brownish secondary color. If the treatment is identifiable, this could affect value, but enhancement may be impossible to detect.
Pricing. According to The Guide, fine-quality demantoid of 1 ct. to 2 cts. can be priced from $700/ct. to $2,000/ct. From 2 cts. to 3 cts., it can run anywhere from $3,000/ct. to $4,000/ct. Extra-fine and larger gems can be priced upwards of $10,000/ct.
Care and cleaning. Hardness is only 6.5, so take the usual precautions with cleaning by washing the dust from the gem before wiping with a cloth. Dust can dull the polished surface.
Bench precautions. When repairing a piece of jewelry set with demantoid garnets, one should be aware not only of the gemstone, but also the provenance of the jewelry. Improper repair of an important estate piece could dramatically lessen the value of the jewelry. As for the garnet, demantoid is softer (and more important) than most other garnets and should be handled with care.
Recommended reading. For more information, see “Russian Demantoid, Czar of the Garnet Family,” Wm. Revell Phillips and Anatoly Talantsev, Gems & Gemology, GIA, Summer 1996.