DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Rhyolitic geode. Chalcedony (red/brown) in a piece of rhyolite rock. The geode has been cut open and polished to show internal structure and to improve its appearance. A geode forms when layers of the mineral silica (silicon dioxide) precipitate from liquid onto the inside of hollows in rock. Banded outer layers are chalcedony, an amorphous form of silica, with impurities causing the colours. The outer layers are red, turning brown towards the central hollow. The final layer (very small here) is pure crystals of quartz (crystalline silica). Rhyolite rock is volcanic. Volcanic rocks often contain such spaces that are suitable for forming geodes. This sample is from Western Australia.
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