opal n an amorphous, usually iridescent, mineral that can be of almost any colour, found in igneous rocks and around hot springs. It is used as a gemstone. Composition: hydrated silica. Formula: SiO2.nH2O (C16: from Latin opalus, from Greek opallios, from Sanskrit upala precious stone)
n any opal of a dark coloration, not necessarily black
n an orange-red translucent variety of opal, valued as a gemstone
n a form of opal having a reddish-brown coloration, (Also called)
n glass that is opalescent or white, made by the addition of fluorides
n a form of petrified wood impregnated by common opal
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