DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Rhyolitic geode. Chalcedony (red) inside rhyolite rock (green). The chalcedony is an amorphous form of silica (silicon dioxide). It is coloured red by small amounts of haematite, an iron oxide. The geode has been cut open and polished to reveal its internal structure and to improve its appearance. A geode forms when layers of the mineral silica precipitate from liquid onto the inside of hollows in volcanic rocks such as rhyolite. This sample comes from the Esterel region of France.
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