Natural Stone Selection – Slate.
Slate is the finest grained foliated metamorphic rock. Slate is derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism.
The shale and clay are subjected to a metamorphic process involving heat and pressure during which cleavage planes are formed. Foliation may not correspond to the original sedimentary layering. Called “slaty cleavage”, the very strong foliation is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression.