Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Chalk is mostly formed from the calcareous skeletons of countless tiny planktonic algae called coccoliths. Blackboard chalk is not real chalk. It is actually gypsum, but often people call it chalk. Blackboard chalk is usually sold in sticks that are about 5 cm long. A single stick of chalk costs about $0.25 and a pack of chalk is priced at $18. Tailors’ chalk is also not real chalk. It is actually talc. Tailors use it to draw on material when they are making clothes.