KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY
Quartz geode. Polished surface of a sectioned quartz geode. Geodes are formed by deposition of silica on the inner surface of rock cavities, which often occur in sedimentary rock containing fossil shells. The thin outer layer of the geode consists of dense chalcedonic silica. Silica precipitates inside this as water passes through, eventually building up to form crystals of quartz. The inner core of this geode is still hollow.
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