Hydrated silica, five to ten per cent water by weight. That water is why opal plays colour, and why it is handled gently: no ultrasonic cleaners, no sudden heat, no long stretches in hot dry sun.
Opal that formed in seams through Queensland ironstone and is cut with its host rock left on. The stone owes its dark face to the iron behind it, and no two seams run alike.
Hardness — 5.5 to 6.5 Care — no ultrasonic, no heat Source — Queensland, Australia
Precious opal from the inland fields — Coober Pedy, Lightning Ridge, Andamooka. White to dark ground with a play of colour that moves as the stone turns.
Hardness — 5.5 to 6.5 Care — no ultrasonic, no heat Source — South Australia, New South Wales
Hydrophane opal from the Welo highlands. It can drink water — the colour may quiet for a day or two after a soaking and returns as it dries. Taken off before washing up.
Hardness — 5.5 to 6 Care — keep dry; remove before washing Source — Welo, Ethiopia
Translucent orange to red, coloured by iron rather than play of colour, though some stones carry both. Most of it comes out of Mexican cantera.
Hardness — 5.5 to 6.5 Care — no ultrasonic, no heat Source — Querétaro, Mexico