Low-grade, regionally metamorphosed rock, which is highly fissile and fine grained. The fissility (slaty cleavage) results from the parallel alignment of numerous fine phyllosilicate minerals (e.g. muscovite and chlorite) induced by compressive tectonic deformation. The smooth, hard, impermeable surface produced when slate is split makes it commercially valuable for roofing, cladding of buildings, and for making such items as billiard-table tops, laboratory benches, and blackboards. Compare phyllite; schist.