Slate Slate 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. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. The foliation in slate is called "slaty cleavage". It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression. When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates
Slate the 20th century. For example, "roof slate" referred to shale above a coal seam, and "draw slate" referred to shale that fell from the mine roof as the coal was removed. Slate is mainly composed of the minerals quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite and, less frequently apatite, graphite, kaolinite, magnetite, tourmaline, or zircon as well as feldspar. Occasionally, as in the purple slates of North Wales, ferrous reduction spheres form around iron nuclei, leaving a light green spotted texture. These spheres are sometimes deformed by a subsequent applied stress field to ovoids,