Slate

Slate is a fine-grained metamorphic rock that splits along parallel planes into thin, durable sheets, a property called cleavage that has made it architecture's classic roofing material for centuries, alongside use as flooring, cladding and writing surfaces. Its low water absorption and resistance to frost damage give a well-laid slate roof a service life measured in generations rather than decades, and its natural gray, green, purple or black coloring, depending on mineral content and quarry origin, needs no applied finish. Wales, Vermont and Pennsylvania have historically been major slate-producing regions supplying roofing across Britain and the northeastern United States.