Corundum is the crystalline form of aluminum oxide and one of the rock-forming minerals. Corundum is naturally clear, but can have different colors when impurities are added. Transparent specimens are used as gems, called rubies if red, and sapphires otherwise.
Corundum has a hardness of 9 on the 10 point Moh's scale, a specific gravity of 4.00 and a hexagonal crystal structure. The oxygen atoms are arranged in a hexagonal close packing, with the smaller aluminum atoms occupying 2/3 of the octahedral gaps. The coordination of the atoms are thus 6:4, compared to 4:2 for quartz, which accounts for its greater hardness despite the Al-O bonds being less covalent.