Opal is hydrated silica and the refraction of light by silica spheres creates a play of colour when viewed from different angles. Fire opal (mainly mined in Mexico) is a variety in which red and yellow colours predominate. The main producer of opals is Australia, although good material is now being mined in Ethiopia. Do not confuse opal with "opalite" - this is a man-made synthetic stone, usually glass. Birthstone for October. Hardness 6 (Mohs).