Corundum

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).

Chemical composition - Al 23
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) / massivegranular (called emery).

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