Chalk is a white, sedimentary rock, composed of calcium carbonate. The calcium carbonate originates from the calcareous liths deposited by blooms of species of coccolithophore phytoplankton. One of the most dramatic outcrops of chalk is along the south coast of England where it forms the "white cliffs of Dover", the Seven Sisters and parts of England's Jurassic Coast, now a World Heritage Site in recognition of its unusual geology.
As a useful commodity, chalk has been replaced by other substances. For example, the "chalk" that is used in blackboard writing or in sidewalk chalk is actually gypsum. So, in common usage, chalk refers to a cylinder-shaped piece of gypsum that is used as a specialized writing implement.