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