Slate, a common metamorphic rock that splits into thin, even sheets. Slate consists primarily of such minerals as muscovite (mica), chlorite, and quartz. Other minerals that may also be present in lesser amounts include hematite, rutile, and tourmaline. Slate is usually gray or black but also occurs in such colors as red, green, brown, and yellow. It is fine-grained rock; the individual grains cannot be seen with the unaided eye.
Slate forms when shale, volcanic tuff, or other fine-grained rocks are subjected to great heat and pressure in the earth's crust. The grains of the original rock are squeezed into flakes by the pressure and new minerals form parallel to the flakes. The slate will split along the flakes, which lie at right angles to the direction of the pressure that formed the slate.
Slate is quarried in blocks and then separated into thin sheets. There are large slate quarries in Pennsylvania, Virginia, and the New York-Vermont area. Sheets of slate are used for roofs, blackboards, flagstones, and billiard-table beds. Crushed slate is used in abrasives, shingles, and pigments.