Corundum also comprises two of the four most precious gemstones, rubies and sapphires. Though generally thought of as blue, sapphires actually come in all colors with the exception of red only because the red variety of corundum is classified as ruby. Pure corundum is actually colorless, different impurities in the mineral give corundum its color, for example, iron and titanium makes it blue and chromium makes it red.

Second only to diamond (10 on the MOH’s hardness scale) and moissanite (9.5 on the Moh’s hardness scale), corundum (9 on the MOH’s hardness scale) is one of the hardest materials on earth. In addition to the formation of beautiful gemstones the hardness of corundum has practical uses, particularly as an abrasive. Corundum is used for grinding optical glass, polishing metals, and also has been made into sandpapers and grinding wheels. This mineral can also be used to make industrial bearings, high end watch crystal faces, and scratch-resistant windows for electronic instruments as well as wafers for circuit boards and other similar products.