Moss agate, close-up. Agate is an aggregate of different forms of silica (chalcedony) within a geode. A geode forms when the silica precipitates from liquid onto the inside surface of an empty pocket in a host rock. Moss agate, also called mocha stone, gets its name from the green-coloured minerals it contains (possibly chromium- or iron-bearing) in a dendritic pattern suggestive of moss. The colours and arrangements of microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process. Magnification: x2.2 when printed 10 centimetres wide.