DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Rhyolitic geode. Chalcedony (red) and crystals of quartz (lining the central hollow) formed inside a piece of rhyolite rock (surrounding structure). The geode has been cut open and polished to show its internal structure and improve its appearance. A geode forms when layers of the mineral silica (silicon dioxide) precipitate from liquid onto the inside of hollows in rock. An amorphous form of silica (chalcedony) forms the banded layers seen, with haematite, an iron oxide, colouring it red. The inner parts form last and are pure crystalline silica (quartz). Volcanic rock, like rhyolite, has spaces suitable for the formation of geodes. This geode is from the Esterel region of France.
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