Corundum
Oxide · Al₂O₃ · also: Ruby, Sapphire
Corundum is aluminium oxide and the second-hardest natural mineral; its red gem variety is ruby and all other colours are sapphire.
What is corundum?
Corundum is aluminium oxide, hardness 9, beaten only by diamond. Trace chromium makes it red (ruby); iron and titanium make it blue (sapphire); and other traces give yellow, pink, and every colour of sapphire. It forms barrel-shaped hexagonal crystals and is used as an abrasive as well as a gemstone.
Properties
- Chemical formula
- Al₂O₃
- Category
- Oxide
- Hardness (Mohs)
- 9
- Crystal system
- Trigonal
- Lustre
- Vitreous to adamantine
- Streak
- White
- Colour
- Every colour; red (ruby), blue (sapphire)
- Cleavage / fracture
- None; parting
How to identify corundum
- →Extreme hardness 9: scratches almost everything, scratched only by diamond.
- →Barrel- or spindle-shaped hexagonal crystals, often with flat ends.
- →High density; glassy to brilliant lustre.
- →No true cleavage.
Where corundum is found
Ruby and sapphire come from Myanmar, Sri Lanka, Madagascar, Thailand and Australia; abrasive corundum (emery) from Greece and Turkey.
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