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. Associated Minerals include calcite , zoisite , feldspars , micas and garnets . Best Field Indicators are extreme hardness, density and crystal habit.