Professional Jeweler Archive: A Horse Tale

September 2000

Gemstones & Pearls: Gemology

A Horse Tale

This inclusion in demantoid garnet baffles gemologists – one of two theories may be the answer to its beauty

Every demantoid garnet has either a hint of this flaxen inclusion or, on occasion, a flamboyant shock of it. If the cutter handling the gem rough is careful enough, the inclusion can be positioned in the center of the gem.

The inclusion, known in gemological circles as a “horsetail,” is prized by gem collectors, particularly when it’s in the center of the gem and easily seen.

What makes up the inclusion remains somewhat of a mystery. In classic texts, horsetail inclusions are identified as byssolite fibers, a form of fibrous amphibole. More recently they’ve been described as chrysotile, a form of fibrous serpentine.

“I would not dismiss either theory outright,” says John Koivula, chief gemologist at the Gemological Institute of America, Carlsbad, CA. “It’s possible both are right.”

Koivula says he hasn’t done any testing of the inclusion himself, but notes that a Raman microspectrometric study was published in the Winter 1999 issue of Gems & Gemology, GIA’s quarterly journal.

Demantoid is a rare garnet from the andradite garnet family. It’s thought to be colored by a combination of iron oxide and chromium. The classic source is along the Bobrovska River in the Ural mountains of Russia.

– by Robert Weldon, G.G.

This 1.34 ct demantoid with a horsetail inclusion in the center comes from the Ural Mountains in Russia.

Copyright © 2001 by Bond Communications