Agate slice. Agate is an aggregate of different forms of silica (silicon dioxide, SiO2) that have formed concentric bands within a geode. A geode forms when the silica precipitates from liquid onto the inside surface of an empty pocket in a host rock. The outer layers are variegated forms of chalcedony (amorphous or non-crystalline silica), with the layers becoming more crystalline towards the centre. Quartz (centre) is the crystalline form of silica. The colours and arrangements of microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process.