Geode in thin section, close-up, viewed in transmitted polarised light. A geode forms when silica (SiO2) 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. Here, a core of quartz (the crystalline form of silica) can be seen. The banded arrangement of crystals are a result of changes in pressure, temperature, and mineral content occurring during the formation process. Magnification: x3.8 when printed 10 centimetres wide.