Rhyolitic geode. Opposite halves of a geode. Geodes form when a hollow in a rock becomes filled with mineral crystals that grow inward to the centre of the hollow. This geode formed inside rhyolite, a volcanic rock formed by the cooling of lava on the Earth's surface. Many geodes contain chalcedony, a term for various cryptocrystalline quartz (silicon dioxide) gemstones, including agate. The colours are derived from impurities in the mineral. This geode was found in Australia.
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