Slate is a low-grade metamorphic rock that forms from shale
under pressure at temperatures of a few hundred degrees or so. Clay minerals in the shale, which originally formed by the breakdown of mica minerals
in igneous rocks, begin to revert back to mica. This makes the rock harder, so that it rings or "tinks" under the hammer, and it results in a strong cleavageslaty cleavageso that slate readily breaks into thin plates. Slaty cleavage doesn't always match the original sedimentary bedding planes; instead it reflects regional tectonic forces during the geologic past. Fossils in the original rock are usually erased, but sometimes they survive in smeared or stretched form.
Slate makes excellent paving stones, long-lasting roof tiles and, of course, the best billiard tables. Blackboards and writing tablets were once made of slate, and the name of the rock has become the name of the tablets themselves.
With further metamorphism, slate turns to phyllite.
See more metamorphic rocks