Polished 'crazy lace' agate, close-up. Agate is an aggregate of different forms of silica (SiO2) within a geode. A geode forms when the silica precipitates from liquid onto the inside surface of an empty pocket in a host rock. Crazy lace agate is an agate in the form of chalcedony (amorphous or non-crystalline silica), its patterns reminiscent of Brussels lace. The colours and arrangements of microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process. The wonderful resulting swirls and intricate patterns make it a popular choice for use in jewellery. This piece was found in Chihuahua, Mexico. Actual width: 15 centimetres.