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Mineral Properties
Slate is a metamorphic rock. It is derived from shale or mudstone that has experienced intense heat or pressure beneath the earth's surface. Slate is made up of parallel foliated plates. This gives it the ability to break smoothly and evenly along its cleavage. The hardness of rocks and minerals is measured using the Mohs scale, which ranges from one to 10 with one being the softest and 10 being the hardest. Slate ranks at 5.5 on the Mohs scale. It is hard enough to be able to scratch glass and steel.
Colors
The minerals it contains determine the color of slate. Red slate is caused by hematite, green by chlorite, blue-gray by sericite, black by carbonaceous materials and yellowish-brown by limonite. Some types of slate are also mottled, streaked or spotted because of its mineral content. The cleavage surface of slate may have either a shiny or dull luster. Cross fractures are typically dull. Slate is an opaque material that does not transmit any light.
Where It Is Found
Some of the most well known deposits of slate are found in the Western Hemisphere. Scotland and northern Wales have been supplying slate for roofing and paving since at least the 16th century AD. The early Celts may have had slate rooftops. The United States has a few substantial slate quarries along its eastern seaboard. There are also deposits found in its western and southern states. The Canadian provinces of Ontario, Nova Scotia and New Brunswick also have quarries. Brazil is famous for its "rusty slate," which contains the splashes of multiple colors on subtle gray.
Uses
Slate has historically been used for shingles and paving tiles. Slate also serves many decorative purposes. The rock makes plates, picture frames, coasters, trivets, beads and jewelry. Slate is also used to make fountains. The relatively flat and receptive surface of slate makes it fairly easy to write on with materials like chalk or charcoal. Slate has been used to make chalkboards for classrooms. Some artists use slate as a natural canvas.