Chalk can be made by stirring together calcium carbonate and clay, or it can be manufactured by molding. Chalk is made mostly of limestone or gypsum.
Extruded chalk is made by stirring calcium carbonate, color pigment, clay and a binding agent to form a doughy material, which is then pressed into cylindrical shapes and cut into sticks of chalk. This type of chalk is dustless, meaning that the dust doesn't travel through the air. It is typically used as blackboard chalk.
Molded chalk is not dustless and is made by mixing dry pigment, water and plaster. The chalk is then molded into sticks and kiln-dried.
Chalk is formed naturally from limestone, a sedimentary rock that is made of calcium carbonate. It also forms from gypsum, a mineral known as calcium sulfate that is formed from the residue of evaporated water.