Geode interior. Thin slice of a geode showing the layers of silica surrounding the central hollow. A second geode slice lies behind the first. A geode forms when layers of the mineral silica (silicon dioxide) precipitate from liquid onto the inside of hollows in rock. Mineral impurities form the colours seen. The opaque silica is amorphous (non- crystalline) silica, named chalcedony. The colours can form bands (brown) or delicate branching tree- like structures (green).