DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Agate geode. Close-up of the polished face of a section through an agate-containing geode. Agate is an aggregate of different forms of silica (SiO2) within a geode. Geodes form when silica precipitates from liquid onto the inside surface of empty pockets in a host rock. This specimen was found in Australia and measures 8x12cm.
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