Slates are metamorphic rocks, most of which began as clay sediments deposited by water. The age of slates varies somewhat, but many were formed among old deposits of rock found deep beneath the surface of the Earth. Slate appears in a wide range of colors and greatly depends upon the mineral composition of the rock. Red slate, for instance, gains its hue from the significant amount of hematite, a crystalline form of ferric oxide, it contains. Green slates, on the other hand, often are comprised of sizable amounts of the green clay mineral chlorite, and dark slates often owe their color to carbonaceous material or fine particles of iron sulfide.