Feature Gems & Gemology, Winter 1985, Vol. 21, No. 4

A Proposed New Classification for Gem-Quality Garnets

Existing methods of classifying garnets have proved to be inadequate to deal with some new types of garnets discovered recently. A new classification system based on the chemical analysis of more than 500 gem garnets is proposed for use in gemology. Chemical, optical, and physical data for a representative collection of 202 transparent gem-quality stones are summarized. Eight garnet species are defined—grossular, andradite, pyrope, pyrope-almandine, almandine-spessartine, spessartine, and pyrope-spessartine—and methods of identification are described. Properties that can be determined with standard gem-testing equipment (specifically, refractive index, color, and absorption spectrum) can be used to identify a garnet as one of the eight species and, where appropriate, more precisely as one of several varieties that are also defined.