Common corundum was formed under a wide variety of geological conditions. In almost any mineral, crystals suitable for gem use occur only under unusually good conditions. Conditions conducive to ideal crystal growth seem even more rare during the formation of corundum than of most other gem minerals, for it is among the most widely occurring but least commonly transparent minerals. The right conditions seem to occur most frequently in the contact metamorphism of certain limestone's that re-crystallize into marble. In the process, the necessary impurities in the limestone, mainly aluminum oxide, are concentrated under conditions that permit them to crystallize as corundum.
Corundum also occurs in a variety of aluminum rich igneous rocks. There are some pegmatite dikes bearing a higher than ordinary percentage of aluminum in which gem quality material was formed. Also, it is occasionally found in very basic igneous rocks. It is associated in North Carolina with dunite, a basic rock composed mostly of olivine (peridot). It is also found in a very basic igneous dike in the Yogo Gulch deposit in Montana. In eastern Canada, corundum is the major constituent in syenite, a rock just slightly less acid than granite. With the exception of the Yogo Gulch mine, the gem corundum deposits of the world are almost exclusively alluvial in nature. In Burma, sapphires and rubies are found in crystalline limestone; usually, however, the concentration is too low to enable the deposits to be worked on a profitable basis.
