Slate is a fine-grained, foliated metamorphic rock formed from shale or volcanic ash through low-grade regional metamorphism, distinguished by its slaty cleavage, which allows it to split into smooth, flat sheets. This property of fissility has made it a classic material for roofing, floor tiles, and billiard tables, and historically it was used as writing slates, giving rise to the phrases "clean slate" and "blank slate." While typically grey, slate occurs in a variety of colors, including purple, green, and cyan, as famously seen in the quarries of North Wales. Composed mainly of quartz, illite, and chlorite, slate forms under strong compression that can reduce the original mudrock's volume by up to 50%, and it should not be confused with shale or schist. Because it forms under relatively low heat and pressure, slate can occasionally preserve delicate fossils, offering a unique glimpse into ancient life.