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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