Agate. Slice through an agate, a type of chalcedony. The brown-black colour is from manganese oxide. Chalcedony is a cryptocrystalline variety of the

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Agate. Slice through an agate, a type of chalcedony. The brown-black colour is from manganese oxide. Chalcedony is a cryptocrystalline variety of the mineral quartz (silica, SiO2). Agates form when crystals precipitate out of solution in the centre of a rock hollow. The crystals gradually fill the hollow to create a geode. Different mineral impurities impart different colours to agates. This agate was found at Shiprock Mountain, New Mexico, USA.

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Photo credit: © DIRK WIERSMA/SCIENCE PHOTO LIBRARY / Alamy / Afripics
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Keywords: agate, cryptocrystalline, crystalline, cut, dioxide, gemstone, geode, impure, impurities, maganese, mineral, natural, oxide, polished, precipitate, quartz, rock, section, sectioned, semi-precious, silicon, sliced, stone, surface