Geode interior section. Polarized light micrograph of a thin section through a geode. Layers of chalcedony (banded) surround an interior of quartz crystals. A geode forms when layers of the mineral silica (silicon dioxide) precipitate from liquid onto the inside of rock hollows. Chalcedony, the amorphous or non-crystalline form of silica, forms the outer layers. The final, central part that forms is the pure crystalline form of silica, quartz. The normal appearance of the chalcedony is coloured by impurities with the pure quartz being colourless. The central quartz hollow is about 2 centimetres across.