[1] Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.

[1] When expertly "cut" by striking parallel to the foliation with a specialized tool in the quarry, many slates display a property called fissility, forming smooth, flat sheets of stone which have long been used for roofing, floor tiles, and other purposes.

Slate is a fine-grained, metamorphic rock that shows no obvious compositional layering but can easily be split into thin slabs and plates.

This is in contrast to the silky cleaved surfaces of phyllite, which is the next-higher grade of metamorphic rock derived from mudstone.

Under a microscope, the slate is found to consist of very thin lenses of quartz and feldspar (QF-domains) separated by layers of mica (M-domains).

Slaty cleavage is fully developed as the clay minerals begin to be converted to chlorite and mica.

These spheres are sometimes deformed by a subsequent applied stress field into ovoids, which appear as ellipses when viewed on a cleavage plane of the specimen.

[14] In the context of underground coal mining in the United States, the term slate was commonly used to refer to shale well into the 20th century.

[16] The British Geological Survey recommends that the term "slate" be used in scientific writings only when very little else is known about the rock that would allow a more definite classification.

For example, if the characteristics of the rock show definitely that it was formed by metamorphosis of shale, it should be described in scientific writings as a metashale.

A series of "slate booms" occurred in Europe from the 1870s until the First World War following improvements in railway, road and waterway transportation systems.

[18] Natural slate, which requires only minimal processing, has an embodied energy that compares favorably with other roofing materials.

In areas where slate is plentiful it is also used in pieces of various sizes for building walls and hedges, sometimes combined with other kinds of stone.

Deposits of slate exist throughout Australia, with large reserves quarried in the Adelaide Hills in Willunga, Kanmantoo, and the Mid North at Mintaro and Spalding.

Parts of France (Anjou, Loire Valley, Ardennes, Brittany, Savoie) and Belgium (Ardennes), Liguria in northern Italy, especially between the town of Lavagna (whose name is inherited as the term for chalkboard in Italian) and Fontanabuona valley; Portugal especially around Valongo in the north of the country.

Germany's Moselle River region, Hunsrück (with a former mine open as a museum at Fell), Eifel, Westerwald, Thuringia and north Bavaria; and Alta, Norway (actually schist, not a true slate).

The roof of St. Patrick's Cathedral in New York City and the headstone of John F. Kennedy's gravesite in Arlington National Cemetery are both made of Monson slate.