Sapphire
Corundum in every colour but red — famed for its blue, and prized for durability.
Also known as: Corundum
beginner Metamorphic and igneous rocks
What it is
Sapphire is gem corundum (aluminium oxide) in any colour other than red, which is classed separately as ruby. Blue sapphire owes its colour to traces of iron and titanium, while chromium, iron and other elements yield pink, yellow, green and the rare orange-pink padparadscha. Like all corundum it is 9 on the Mohs scale, second only to diamond, and exceptionally tough for rings.
Classic sources include Kashmir (famed for a velvety cornflower blue), Myanmar, Sri Lanka, Madagascar and Australia. Heat treatment to improve colour is routine and stable; more aggressive diffusion treatments exist and should be disclosed. Star sapphires show a six-rayed asterism caused by needle-like rutile inclusions.