Guest Author - Susan Dorling

‘Ask Our Gemology Expert’ is a regular feature at the Jewelry Collecting site, kindly provided by our guest contributor, Barbara W. Smigel, PhD., Graduate Gemologist, GIA. As site Editor, my goal is to make the Jewelry Collecting site as comprehensive and interesting as possible for my visitors. Barbara’s expert guidance in this series, and her major contributions to our ‘Everything You Ever Wanted to Know’ series about gemstones, is greatly appreciated and an invaluable learning tool for myself and others who collect gemstones and gemstone jewelry. In fact, the more I learn, the more fascinated I become by gemstones themselves, and their exciting role in jewelry design. Thanks, Barbara!

If you have a question for Barbara, please send an email using our Contact Form with subject line, Ask Our Gemology Expert. And remember, if you would like to learn more about gemology and gemstones check out Barbara's free gemology course, Online Gemology Course. You'll love this fascinating and fun way to learn about gemology.
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This question was sent to Barbara by Lauren in Massachusetts.

Dear Barbara,

I have always wondered what the word "grossular" means when describing a grossular garnet--and why are these stones usually green?...Lauren, Massachusetts

Dear Lauren,

The name is from the Greek "grossularia" or gooseberry. This is a reference to the light yellow-green color of the first type of grossular garnet discovered (an opaque, non-gem type). Although the more valuable and available members of this group are green, they are not really the most common, color-wise. The colors range from the colorless of pure grossular (quite valuable to collectors) to yellow, pink, orange, brown and black, each created by the presence of various chemical elements. Some other well known members of this same group are "hessonites -- orangey brown grossulars and Tsavorites -- medium to dark green grossulars.

Grossular garnets often have a particular inclusion which is an identifying characteristic: called "treacle". Under a microscope, swirled patterns are created by tiny included crystals, generally diopside. Hardness ranges from 6.5 -7.5 with an average specific gravity of 3.6. Generally it forms by contact metamorphism and only in localized deposits. Major sources for grossular garnet are Sri Lanka, Pakistan, Russia, Canada, USA, Mexico, Africa, Australia and Brazil.

Hydrogrossular garnet, an opaque to translucent green to pink closely related gemstone, has a long history in the gem trade usually under the name of "Transvaal jade".

Garnets belong to the isometric crystal system and commonly grow in a distinctive well developed crystal form known as a dodecahedron (triangular-shaped faces). Natural garnets are a complex "solid solution series" of gemstones. This means that the various species share a crystal structure and an infinitely grading series of generally similar chemical formulas. Gemologists divide them into the calcium bearing "ugrandites", named after the initial letters of the three calcium species: uvarovite, grossular, and andradite, and the calcium-free pyralspites, named after: pyrope, almandine, and Spessartite.

Aluminum, iron, and chromium substitute freely in the ugrandites whereas iron, magnesium, and manganese substitute readily for one another in the pyralspites. All gradations between the pure species are possible and as most gemologists do not have at their disposal a means for precisely determining the chemical composition of a garnet, we rely on color and refractive index for classification.
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Gemological Properties of Grossular Garnets

Makeup - Calcium aluminum silicate
Crystal System - cubic
Hardness - 7.25
RI (Refractive Index) - 1.73 - 1.75
Density - 3.65
Dispersion -.027
Polish Luster - vitreous to resinous
Cleavage - none