The red variety of corundum is known as ruby and all the other colors of corundum are known as sapphire . Corundum is the second hardest natural mineral known to science. The hardest mineral, diamond is still four times harder than corundum. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.
Color is highly variable.
Luster is vitreous to adamantine. Crystals are transparent to translucent. Cleavage is absent, although there is parting which occurs in three directions. Fracture is conchoidal. Streak is white.