Rhyolitic geode Cut surface of a geode displaying the host rock rhyolite green chalcedony blue white and haematite red Geodes form when a hollow in a rock becomes filled with mineral crystals that grow inward to the centre of the hollow Rhyolite is a volcanic rock formed by the cooling of lava on the Earths surface Chalcedony is a term for many cryptocrystalline quartz silicon dioxide gemstones including agate The colours are derived from impurities in the mineral Haematite is a form of iron oxide This geode came from the Esterel region on the Mediterranean coast in France