A cut and polished fire opal. Opal is composed of microspheroids of hydrous silicon oxide. It is the refraction of light within these spheroids which gives the mineral its iridescent, mother-of- pearl-like sheen. Opal never occurs in a crystaline form, but as small veins, globules and crusts. Several other varieties occur; colourless, milky white, blue, red (fire opal) and black types and also a type of fossilised wood know as wood opal. Black and fire opals are particularly valued as gemstones.
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