Slate is a fine-grained metamorphic rock that derives from sedimentary rock and shale deposits. The main constituents are quartz, chlorite, mica and calcite, with some also containing trace amounts of metals. Slate is formed when ocean or riverbed sediments are compressed and heated by the earth’s crust.
This heat and pressure produces the pronounced partings (cleavage) that give slate its distinct layered characteristic.
The color of slate is determined by its chemical and mineralogical composition. The colors visible in many slates are the result of the stone splitting along natural layers and exposing the metals to the atmosphere, which then oxidize (rust).
Today, most slate is sourced from India, China, Brazil, Wales and in some areas of the Eastern U.S. (mainly Vermont and Pennsylvania).