Opal
A hydrated silica gel famous for play-of-colour — soft, water-bearing and unlike any crystalline gem.
Also known as: Hydrated silica
intermediate Sedimentary silica deposits
What it is
Opal is a hydrated, non-crystalline (amorphous) form of silica, SiO₂·nH₂O, containing up to about 20% water by weight. It is soft and fragile at 5.5 to 6.5 on the Mohs scale and can craze or crack if it dries out or is heated. Precious opal's celebrated play-of-colour comes from an ordered array of microscopic silica spheres that diffract light into shifting spectral flashes.
Australia has historically produced most of the world's fine opal, including white, black and boulder opal; Ethiopia is now a major source of bright "hydrophane" opal that can absorb water. Common opal without play-of-colour is called potch. Opals are best protected from heat, dehydration and sharp knocks.