Al₂O₃

These samples of corundum are displayed in the Smithsonian Museum of Natural History. Corundum is a oxide mineral of aluminum with the composition Al₂O₃. The sample at left is about 6 cm across and is from Franklin, New Jersey.

Pure corundum is colorless, but the addition of a small impurity of chromic oxide can make the precious stone ruby (red) and the addition of titanium oxide can make sapphire (blue and other colors).

Corundum and impure corundum called emery are used in making abrasive cloths and wheels.

This sample of corundum is from Franklin, New Jersey and is about 10 cm across.