Corundum is the only mineral on the Mohs scale assigned a hardness of 9.
It is a crystalline form of aluminium oxide, with the chemical formula Al₂O₃. Corundum’s tightly bonded structure makes it exceptionally resistant to scratching, second only to diamond on the traditional ten-point scale.
Ruby and sapphire are gem-quality varieties of corundum. Chromium commonly gives ruby its red color, while sapphire can occur in blue and many other colors because of different trace elements.
Corundum is also useful industrially as an abrasive. Synthetic corundum is manufactured for grinding, polishing, cutting, and other applications where hardness is valuable.