A gem made of water and light — the one stone that holds a rainbow. Hydrated silica, laid down by water and lit by the orderly grid of microscopic spheres inside it. October birthstone · national gemstone of Australia.
Black Opal — Lightning Ridge, NSW
Opal isn't a crystal at all. It is hydrated silica — microscopic spheres of the same stuff as quartz, left behind as water dried inside the rock. Where those spheres settle into an orderly grid, light bends between them and shatters into the flashes called play-of-colour. Where they don't, you get plain "common" opal with no fire.
Each kind is named for its body tone — the background the fire plays against — and for the field that made it famous.
ClassMineraloid · SiO₂·nH₂O
Hardness5.5–6.5 Mohs
Waterup to ~20%
Usual cutCabochon
ClassesPrecious · Common
BirthstoneOctober
Where the fire comes from
Precious opal is a 3-D grid of sub-microscopic silica spheres. Light diffracts between them and splits into spectral colour — sphere size sets the hue: about 0.1 µm flashes violet, 0.2 µm flashes red, the sizes between fill the rainbow. — GIA
No play-of-colour — the spheres are not ordered. Still true opal, just without the fire. worldwide
Mostly water — treat it gently. Opal can hold up to a fifth of its weight in water and may craze or crack if it dries out or is heated. Hydrophane opal absorbs liquids and shifts in look until it dries. Keep opals from heat, dehydration, and harsh chemicals.
Illustrations are stylised, not photographs; colours and patterns are typical of each kind, not to scale. General educational guidance — value is not financial advice.