ALFRED PASIEKA / SCIENCE PHOTO LIBRARY ALFRED PASIEKA / SCIENCE PHOTO LIBRARY
Opal. A piece of blue, iridescent 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 sheen. This is enhanced by reflections from tiny conchoidal fractures (crazing) which occur on exposure to the air. Opal never occurs in a crystalline form, but as small veins, globules and crusts. Several types occur; colourless, milky white, hazy blue, red (fire opal) and black. There is also a type of fossilised wood know as wood opal. Black and fire opals are particularly valued as gemstones. This specimen was found in Queensland, Australia.
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