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.
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.