Ruby is the red gem variety of the mineral corundum.
Corundum is aluminium oxide, with the chemical formula Al₂O₃. Pure corundum is colourless, but trace elements create its gem colours. Chromium produces ruby’s characteristic red, while corundum of other gem colours is generally called sapphire.
The finest rubies can show a vivid “pigeon blood” red and may display a six-rayed star caused by aligned inclusions. Heat treatment is common in the gem trade because it can improve colour and clarity. Natural untreated stones with strong colour and good transparency are especially valuable.
Ruby is extremely hard, ranking 9 on the Mohs scale, second only to diamond among common natural minerals. It is not the same mineral as red spinel, although the two were historically confused. Famous ruby sources include Myanmar, Mozambique, Sri Lanka, and Thailand.