Blackboard and sidewalk chalk can be made from either calcium carbonate (CaCO3) or calcium sulfate, also known as gypsum (CaSO4. Calcium carbonate is formed by the slow underwater compression and accumulation of the calcite shells of single-celled animals called coccolithophores. Gypsum is found in the form of sedimentary rock in evaporite beds and is deposited by sea and lake waters, hot-water springs and volcanic vapors.
Most of the chalk used today, particularly the type used for sidewalk art, is made from dyed and formed gypsum. Chalk made from gypsum is easier to use than the type made from calcium carbonate. Gypsum is also a raw material found in greater abundance and is used for a wide variety of industrial purposes.