Slate

Also known as lousa, slate is a homogeneous sedimentary rock that is fine grained, opaque, resistant, occurs in a variety of colours and textures and is easily separable into parallel sheets (following the direction of the cleavage). It is primarily composed of argillaceous minerals and laminar mica, which give it a typically smooth structure and sheet jointing (foliation). This makes it suitable as a material for roof tiles, wall claddings and pavements, depending on the type of finishing, in a wide array of construction types.  Its impermeability and beauty, and its resistance to atmospheric agents, show why it has been in use since antiquity, making slate a material with almost limitless possibilities.