Slate is the fine-grained metamorphic rock that commonly splits into thin sheets. It forms when shale or another clay-rich sedimentary rock undergoes low-grade metamorphism, causing microscopic clay minerals to realign into slaty cleavage.
Slaty cleavage allows slate to break into smooth, relatively flat plates, even though the original sedimentary layering may run in a different direction. This property made slate valuable for roofing tiles, flooring, writing tablets, blackboards, and billiard tables. Slate may be gray, black, green, purple, or red depending on its minerals and impurities.
Schist forms at a higher metamorphic grade and has larger visible mica grains. Phyllite is intermediate between slate and schist and often has a silky sheen, while gneiss is coarser and commonly shows compositional banding. Slate is therefore identified by its very fine grain and strong cleavage, not merely by any flat layered appearance.