Slate

Slate is a fine-grained homogeneous sedimentary rock composed of clay or volcanic ash which has been metamorphosed (foliated) so as to develop a high degree of fissility or slaty cleavage which is usually at a high angle to the planes of stratification. This high degree of fissility makes the better grades of slates an extremely useful roofing material (although its use has fallen in favor of synthetic and manufactured substitutes). The finest slates in the world come from Wales, Britain.

See also: List of minerals