Slate is a fine-grained rock that can easily be split into thin, durable sheets. It consists mainly of grains of mica and quartz, plus smaller amounts of chlorite, hematite, and other minerals. Most slate is gray to black in color, but the rock may be red or purple, depending on its mineral content.

Slate is a metamorphic rock. Most slate is formed below the earth's surface by changes in the makeup and appearance of shale, a sedimentary rock. Shale consists of clay and fine particles of quartz. Heat from deep in the earth changes some of the clay in shale into mica and chlorite. Slate results when pressure created chiefly by mountain-forming movements in the earth's crust squeezes the mica and other minerals into parallel layers.