corundum, geology.11, gems

Based on mineralogic and geologic research on corundum occurrences in Russia, and published data on corundum localities occurring elsewhere, it is emphasized that regional metamorphism of different rock types (e.g., marble, gneiss) is the major factor controlling the formation of corundum deposits and determining the quality of the gems they contain. The chemical and physical properties of the metamorphic host rocks ultimately determine the characteristics of gem corundum. The sapphires from Kashmir and rubies from Myanmar, which formed in metamorphic environments, are cited as examples. The superb gems from these deposits had the appropriate chromophoric elements available during their formation, under the proper metamorphic conditions. Characteristic mineral inclusions are considered indicative of specific genetic types of corundum, and, for alluvial stones, the inclusions can be used to infer the origin of the gem corundum. Gem corundums originating from other geologic sources (e.g., igneous or metasomatic) are likely to be of lower quality than those of metamorphic origin.