PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Agate is a variety of chalcedony with multi-coloured concentric banding. Chalcedony is a cryptocrystalline (minute crystals) 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. This sample, from France, is 11 centimetres wide.
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