DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Rhyolitic geode. Chalcedony (red) and crystals of quartz (central hollow), inside rhyolite rock (the surrounding round structure). This geode has been cut open to show its internal structure, and polished to improve its appearance. A geode forms as layers of the mineral silica (silicon dioxide) precipitate from liquid onto the sides of hollows in rock. An amorphous form of silica (chalcedony) forms the banded layers seen, with impurities for colouring. The final inner layers that precipitate are crystalline silica (quartz). Rhyolite rock is volcanic. Volcanic rocks often contain such spaces that are suitable for forming geodes. This sample is from the Esterel region of France.
Model release not required. Property release not required.