Slate is mostly clay, but under extreme pressure, the clay can transform into mica. Its colour is largely determined by the amount of iron it contains, but it is typically grey. Slate typically forms in basins between plate boundaries. It is a natural product that can endure fire, hail, acid rain, hurricane-force winds, insects, and just about anything else Mother Nature can throw at it. Because slate is so long-lasting, it eliminates the need for re-roofing every 10–30 years. Some slate roofs have been in use for hundreds of years!
Slate is used for a variety of purposes, including snooker tables, roofing, gravestones, flooring, and garden decorations.