The garnets, a large and rather complex family group, may vary considerably in their chemical composition, physical and optical properties.
For the gemmologist this means that there is an additional step in the identification process. Firstly he must determine if the properties of the unknown place it within the garnet group?
Secondly to which species does it belong and then finally what variety is it?
Over the years garnets have been subjected to diverse classification schemes by mineralogists and gemmologists. While mineralogists afford recognition to dozens of garnet species those that are most familiar to gemmologists are pyrope,
almandine, spessartite, grossular, andradite and hydrogrossular.
Garnets are silicates, the chemical formula for a pure, end-member pyrope being: Mg3Al2(SiO4)3. In purplish almandine garnet the magnesium is replaced by iron: Fe3Al2(SiO4)3. In the case of the popular variety, trade named “Rhodolite”, approximately 50% of the magnesium has been replaced by iron.
Since a single garnet specimen may consist of molecules of several garnet species there are numerous compounded names. Examples of this would be “Pyralspite” a combination of pyrope, almandine and spessartite, “Pyrandine” pyrope and almandine, “Spandite” spessartite and andradite, “Grandite” grossular and andradite, “Ugrandite” uvarovite, grossular and andradite.
Chrome comes into play in chrome-pyrope garnet, seen as an important indicator mineral for diamond prospectors, it may also cut very fine, intense slightly-purplish red stones.
The most commonly used varietal names include the demantoid and topazolite varieties of andradite and the hessonite and tsavolite varieties of grossular.
Certain trade names have also come into general usage for example “Rhodolite” for a rhododendron red combination of pyrope and almandine, “Tsavorite” for tsavolite, “Malaya” (Tanzanian pyralspite) and “Mandarin” (for Namibian spessartite).
The massive South African hydrogrossular garnet is probably better known by the trade name “South African jade”. A bright green grossular from Merelani Tanzania was marketed as “Merelani Mint”.