DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Fire agate. Agate is a microcrystalline variety of the mineral quartz (silicon oxide). Most agates occur as nodules in volcanic rocks where cavities were filled by silica-rich solutions. As the solutions evaporated, silica deposits built up in regular layers upon the walls. The presence of impurities in the layers can produce a wide range of colours and patterns. The brilliant, iridescent appearance of fire agate is thought to arise from alternating layers (Schiller layers) of silica and iron oxide.
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