Ruby
The red variety of corundum — second only to diamond in hardness, and coloured by chromium.
Also known as: Red corundum
beginner Marble and basalt-hosted deposits
What it is
Ruby is gem-quality corundum (aluminium oxide, Al₂O₃) coloured red by traces of chromium. It shares corundum's hardness of 9 on the Mohs scale, making it one of the most durable gems for everyday wear. The most valued colour is a saturated "pigeon's blood" red; chromium also causes many rubies to fluoresce, deepening their glow in sunlight.
Historically the finest rubies came from Mogok in Myanmar (Burma), with important deposits also in Mozambique, Thailand and Sri Lanka. Heat treatment to improve colour and clarity is extremely common and generally accepted in the trade, though untreated stones command a premium. Any red corundum is a ruby; other colours of corundum are called sapphire.