Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering. Slate is frequently grey in color, but also occurs in a variety of colors even from a single locality.
Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%. Its low tendency to absorb water also makes it very resistant to frost damage and breakage due to freezing. Slate tiles are often used for interior and exterior flooring, stairs, walkways and wall cladding. Slate tiles were used in 19th century UK building construction (apart from roofs) and in slate quarrying areas such as Bethesda, Wales there are still many buildings wholly constructed of slate. Slates can also be set into walls to provide a rudimentary damp-proof membrane. Slate is also used in pieces of various sizes for building walls and hedges, sometimes combined with other kinds of stone. In modern homes slate is often used as table coasters.