Green Gem Garnets

Spread the word: Garnet birthstones don’t have to be red. (See “Jewel of the Month: Gem Garnets,” JCK, November 2000, p. 99.) So climb out of the rut and show your customers the green. There are two important transparent green garnets from which to choose: tsavorite grossularite and demantoid andradite. Look carefully at the two, and you can see some subtle color differences. Demantoid, if it has both the right color and the proper cut, can also show off its special powers of dispersion.

History and romance. Demantoid has a longer history than tsavorite, but only by a hundred years or so. Demantoids were discovered in the mid-1800s in the Russian Ural Mountains and gained popularity in the Art Nouveau, Edwardian, and early Art Deco periods. Demantoids were mined there until the Bolshevik Revolution of 1919, after which the mines were closed. Continuing wars and politics kept the mines shut. No other significant demantoid deposits have ever been uncovered, although there are small quantities coming from Mexico, Namibia, Arizona, China, and Sri Lanka. Up until the 1990s, most jewelers trying to locate a demantoid looked in private collections or period jewelry.

But the past decade has seen a relaxation of political control of the mining areas, and the Ural region is once again producing gemstones. New deposits of relatively steady-albeit small-supplies of demantoids are helping the green garnet regain a small portion of its once very popular standing.

The newcomer, tsavorite, was uncovered just 30 years ago near Tsavo National Park in East Africa by geologist Campbell Bridges of Bridges Exploration in Nairobi, Kenya. Since his initial discovery, Bridges has helped uncover the green gem in a number of mines along a mineral belt that stretches across the border between Tanzania and Kenya, from the tanzanite mines of the Meralani Hills to the game preserve of Tsavo. Soon after he discovered the gem, Bridges presented one of the Tanzanian transparent green grossularites to Henry Platt, then president of Tiffany & Co. It is reported that Platt coined the name “tsavorite” for the green gem.

Color variations. Both demantoids and tsavorites can be found in rich, saturated pure greens. Some have a secondary yellow hue. Demantoids show brownish tints, and a few tsavorites appear to have an emerald-like bluish secondary tint. While the preferable color for tsavorite is the darker and more pure green hue, demantoid should have a slightly lighter tone and yellowish tint. This tone and color combination allows for demantoid’s display of dispersion.

Qualities. Demantoid’s dark green color is quite beautiful, but its gemological importance comes not from how green it can be but from its ability to split light into spectral colors. Demantoid gets its name from the fact that it can appear as fiery as a diamond: “demantoid” means “diamond-like.” Not many gems have this dispersive quality.

To release this dispersion, the gem cutter must consider two key factors: angles and body color. As in a diamond, the facets must be placed at the proper angles, which help bend the light coming out of the gem. The facets act as a prism and split the light into rainbow colors. Just as important, the body color must not be so saturated that it absorbs the light that should be released as spectral fire. Thus, in contrast to most other colored gems, the best demantoids have a medium-light to medium-dark tone with only moderately strong to strong saturation, rather than a dark tone with vivid saturation.

Because tsavorite’s beauty is not dependent on releasing dispersion, the more saturated the color, the better. Tsavorite should display a medium-dark to dark tone with vivid saturation.

Value. Tsavorite is the more typical green gem-the higher qualities show few eye-visible inclusions. Demantoid, on the other hand, is best known for a unique inclusion: a bundle of fibers that fan out, much like a horse’s tail. If these inclusions are present in the rough crystal, the cutter doesn’t look to remove them but keeps them in the final faceted stone.

The latest prices. Tsavorites typically do not come in large, 3-ct.-plus sizes. Fine-quality 2-ct. to 3-ct. tsavorites range in price from $900 to $1,200 per carat. Demantoids of similar size and quality can be priced at $3,000 to $4,000 per carat.

Enhancements. As with all other garnets, no enhancements are known to affect tsavorite or demantoid.

Bench care and cleaning. Because some garnets tend to show a little stress, bench jewelers should avoid going directly from steam cleaning to a cold rinse. Otherwise, your staff and customers should take normal precautions when caring for garnets. Since its hardness is somewhere near that of quartz (7-7.5), dust-which is basically silica/quartz-should be washed off instead of wiped off, since dust could scratch the surface of the stone.

Recommended reading. For more information on gem garnets, see the following:

Naming Gem Garnets, by W. Wm. Hanneman, Ph.D., Hanneman Gemological Instruments, 2000.