What is slate?

Slate is a fine-grained, homogeneous, sedimentary rock composed of clay or volcanic ash which has been metamorphosed in layers. Slate can be made into roofing slates, because it has two lines of breakability: cleavage and grain. This makes it possible to split slate into thin sheets. Fine slate can also be used to hone knives.

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.