Chalk is calcium carbonate and has the same chemical composition as limestone, marble, ground calcium carbonate and precipitated calcium carbonate. Chalk, like the other types of calcium carbonate, is in the form of calcite, which is a crystal.
Chalk derives from shells of coccoliths or algae. When these marine organisms die, their shells build up deposits that are mined to make chalk. The sticks of chalk used to write on blackboards in classrooms are made from the shells of these tiny organisms.
Chalk sticks are made by pulverizing natural chalk and adding water, various colors and clay as a binder. Artists first used chalk thousands of years ago, evidenced by the chalk drawings on cave walls.