Corundum

a well known mineral. It is a compound of aluminum and oxygen. It is a compound of aluminum and oxygen. It is hard, ranking next to the diamond. There are three recognized types of corundum. The first is the gem called the sapphire. Five varieties are known. A second type is corundum proper. It differs from the gems only in lack of bright color. It is used as a polishing material. The third form is called emery, which is merely an impure corundum. There is an important corundum deposit north of Kingston, Ontario. Supplies are obtained also from New Jersey and the Carolinas. See Sapphire; Emery; Grindstone