Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment

 

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February, 2003

Grossular Garnet

This essay is about the non-Tsavorite members of the grossular family of garnets. The most valuable and well known member of the whole group is rich green grossular, marketed as Tsavorite, and has been covered separately (see Gem of the Month Archive: February, 1998). Despite the well deserved fame of the glamorous dark green stones, the other members of this highly variable group are interesting and desirable in their own rights. Grossular garnets range from colorless (rare) through shades of yellow and orange. The orange to brownish orange pieces, primarily from Sri Lanka and India go by the variety name "hessonite". The rich color is imparted by traces of iron in their chemical makeup. Hessonites usually have grainy and/or swirly inclusions which create a characteristic internal picture known as "treacle". Lighter oranges, yellows and peachy colored stones often come from famed Canadian deposits in Alberta. The few colorless pieces, usually scooped up eagerly by collectors, generally originate in California. African light green stones, colored by chromium and/or vanadium, which aren't in the color range of Tsavorites are sometimes called "Merelani mint" or just green grossular.

A closely related species is the translucent to opaque hydrogrossular garnet, of Mexico and various African countries. These stones range from green through multicolored and have been long used as jade subsitutes in cabs and carvings, sometimes under the name "Transvaal Jade".

Grossular garnets are excellent gemstones. With a hardness of 7.25 and good toughness they are wearable in any type of jewelry and require no specialized care. Although it's unusual to find grossular garnets totally flawless at 10x, the inclusions are generally balanced by their lovely colors and excellent brilliance and luster. There are no known enhancements or synthetics to worry about. So there's no reason not to branch out from the red and green garnets and explore this interesting group of gemstones.


Value Considerations

There are few special value considerations for grossulars. All the usual factors apply: saturation of color, size and beauty of cutting being the most important. A rarity premium applies to some of the colors, like colorless and peach, and standards for clarity need some relaxation considering that this group like emeralds and South American rubellites rarely are completely clean.
Gemological Properties:

Makeup: Calcium aluminum silicate

Crystal System: cubic

Hardness: 7.25

RI: 1.73 - 1.75

Density: 3.65

Dispersion: .027

Polish Luster: vitreous to resinous

Cleavage: none


Stones Currently Available:

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Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment