Polished Esterel rhyolite agate, close-up. Agates form when silica (SiO2) precipitates from liquid onto the inside surface of an empty pocket in a host rock. The outer layers here are of the geode (the rock encasing), recognisable by the characteristic green hue of rhyolite. In the centre is a core of blue/white chalcedony (amorphous or non-crystalline silica), stained with the vivid reds of haematite. The colours and arrangements of microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process. This geode is from the Esterel region of France. Magnification: x2.2 when printed 10 centimetres high.