Professional Jeweler Archive: It's a Garnet, Darn it!

July 2001


It's a Garnet, Darn it!

The challenge of determining what is in a diamond

While gem cutters often eliminate inclusions in a stone, they sometimes turn them into a featured attraction. Such is the case with this extremely rare diamond with a red garnet inclusion. Such gems are considered highly collectible.

This type of inclusion can’t be analyzed directly because it’s encased in the diamond. So to determine what it is, first study the shape. If visual tests using different forms of illumination prove inconclusive, additional non-destructive gemological tests need to be performed. For example, if all but ruby and garnet have been eliminated as a possibility, test to see whether the crystal is pleochroic. Rubies (which exhibit pleochroism) in diamonds are extremely rare, rarer even than red garnets (which don’t exhibit pleochroism).

Rubies are doubly refractive, meaning that when light enters them, it’s split into two polarized rays traveling at different velocities. Garnets are singly refractive, as are diamonds. By looking at an inclusion through a gemological tool called a dichroscope, you can distinguish two or more pleochroic colors when looking at transparent, colored, doubly refractive stones. Singly refractive stones, like this garnet, don’t exhibit pleochroic colors.

– by Robert Weldon, G.G.

A red garnet crystal in a diamond, positioned at the center of the stone, is reflected throughout the gem. Photo courtesy of the Gemological Institute of America. Photo by Robert Weldon.

Copyright © 2001 by Bond Communications