Which is the major component of blackboard chalk?

I often think about the white cliffs of Dover, when someone mentions chalk

Player #53553731: Calcium carbonate is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of eggshells, gastropod shells, shellfish skeletons and pearls. It has nothing to do with soda pop. .