Natural chalk is a common form of calcium carbonate. Unlike other, chemically similar minerals, chalk deposits are laid down by living organisms known as coccolithophores. These microscopic animals thrive in the warm, shallow seas that proliferated in the Cretaceous Period, which derives its name from the Latin word for chalk.
Natural chalk has been used in its raw form by artists for as long as humans have been etching cave walls. Sometimes impurities are found in the chalk bed, which lend various colors to the chalk. Commercial chalk is usually derived from natural chalk, but with various additives to enrich the color and give it greater strength.
Commercial chalk, of the kind most often used in classrooms, is made from ground chalk powder that has been hydrated and mixed with clay for a firmer consistency. Dyes can be added to give the chalk a desired color. Black chalk, for example, usually contains carbon. Red chalk is often made with ferric oxide, which is a mineral form of iron and oxygen. Some "dustless" varieties of chalk are sold as sticks that have been dipped in a vat of artificial shellac that binds and seals the stick's surface and keeps the chalk from shedding.