Slate is a fine-grained, homogeneous, sedimentary rock composed of clay or volcanic ash which has been metamorphosed (foliated) in layers (bedded deposits). Slate can be made into roofing shingles (‘roofing slates’ in the United Kingdom), because it has two lines of breakability: cleavage and grain. This makes it possible to split slate into thin sheets.
Slate is mainly composed of quartz and muscovite (a mica), often along with biotite, chlorite, and hematite, or, less frequently, apatite, graphite, kaolin, magnetite, tourmaline, or zircon.
Some of the finest slates in the world come from Portugal, Wales in the United Kingdom, the east coast of Newfoundland, and the “Slate Valley” of Vermont and New York.