Chalcedony is a form of silicon dioxide, composed of delicate intergrowths of quartz and morganite. It is a dense, semi-transparent mineral occurring in a large range of colour varieties. Pure, uncontaminated chalcedony appears homogeneous and is white, grey or blue. When illuminated from the back, it may display red hues. Chalcedony can be found in weathering volcanic rocks, but also in sedimentary ones, often together with agate. In magmatic or metamorphic rocks chalcedony is rather rare and only forms veins in fissures that have been percolated by warm, silica-rich brines. Sporadically, chalcedony can be found in fossils, as a petrifying agent.