Corundum is the second hardest natural mineral known to science (1/4 the hardness of diamond). Gem varieties are sapphire and ruby.

Corundum may occur on a large scale in some pegmatites. It is also found in silica-poor hornfelses (a contact metamorphic rock).

corundum (sapphire)

Chemical composition - Al 2O 3
Hardness - 9
Specific gravity - 4+
Transparency - Transparent to translucent
Colour - Highly variable - white or colourless, blue, red, yellow, green, brown, purple, pink
Streak - White
Lustre - Vitreous to adamantine
Cleavage/fracture - Non-existent / conchoidal
Crystal habit/mode of occurrence - Prismatic (six-sided barrel shape that may taper into a pyramid, hexagonal prisms and blades) / massive, granular (called emery)

Other specimens - Click the thumbnails to enlarge