Relatively porous and soft, shale is able to readily take up and release minerals and organic material. Thus, the composition of shales is quite various, heavily depending on the surrounding environment. Ferruginous shale is a type of shale that is rich in iron. The rock typically exhibits a reddish hue if the iron present is ferric or a green tint if the iron is ferrous. Dark gray or black shale, however, often results from the presence of significant amounts of organic matter. Some organically rich shales also contain kerogen, a chemically complex mixture of hydrocarbons that yields oil when heated. These oil shales have periodically been utilized as a fuel source, but are not yet as practically useful as crude oil deposits.