Slates

Slates are a group of fine-grained metamorphic rocks derived from mudstone or shale that have undergone low-grade regional metamorphism. They typically display slaty cleavage, a zone of weak plane foliation that allows them to split into thin, durable sheets. Mineral constituents commonly include muscovite (mica), chlorite, quartz, and plagioclase, with carbonaceous material and accessory grains giving a range of colors from gray and blue-gray to green or purple. Slates form under relatively low temperatures and pressures when shale is buried and subjected to tectonic forces, often in what are known as slate belts along orogenic zones. Prominent producing areas include North Wales, Spain, India, Brazil, and parts of North America.

Because of their cleavage and durability, slates have long been used as building and finishing stones. Roofing slates, floor tiles, and interior cladding are common applications; slate’s resistance to weathering and its ability to take a sharp edge has also made it suitable for school slates and chalkboards in the past. Other traditional uses include cemetery tablets, countertops, and decorative stones.

In modern usage, slate also refers to a slate of candidates—an organized listing of people standing for election or for appointment by a group. In film and television production, a slate (or clapper) is the board used to mark and synchronize takes.

The term should be interpreted according to context: rock slate when discussing geology, building materials when referring to construction, or the various metaphorical senses when discussing lists or production tools.