Slate is quite abundant, fine-grained, and is formed by metamorphism not too intense, at relatively low temperatures and pressures. It is usually thought that the blackboard comes from the metamorphism of the clays (lutites), although you can also produce slate from deposits of volcanic ash. It is a rock of variable color, although predominantly gray and black, which has a foliated texture. A very strong foliation is called "slaty cleavage". It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression. When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates will form smooth flat sheets of stone which have long been used for roofing, floor tiles, and other purposes. In China, a traditional use of slate is the manufacture of ink stones, richly carved, which prepares the ink for calligraphy.