Slate is a metamorphic rock formed when shale is compressed and baked.  The metamorphic process causes the the crystals to orient in the same direction, creating a planar structure in sheets of various thickness.  The planes can be separated into flat sheets.  Some types of slate can be percussion flaked provided the sheets are relatively thick and internally fused.  Most slate, however, is unsuitable for refined flaking because the stone splits along the planes rather than where the stoneworker wants the flakes to go.  Well-fused slates can be hammer dressed and are soft enough for grinding, and slate was often used for making sculptures, ornaments, and edge-ground axes.  In recent times, slate with pronounced planar structure was split into sheets to make pavers and roofing tiles.