slate

Fine-grained, clayey metamorphic rock that splits readily into thin slabs that have great tensile strength and durability. Some other rocks that occur in thin beds are improperly called slate because they can be used for roofing and similar purposes. True slates generally split not along the bedding plane but along planes of cleavage that may intersect the bedding plane at high angles. Slates may be black, blue, purple, red, green, or gray. Slate may be marketed either as dimension slate, used mainly for electrical panels, laboratory tabletops, roofing, and flooring, or as crushed slate, used on composition roofing, in aggregates, and as a filler.