According to Discovering Fossils, chalk occurs naturally in nature as calcium carbonate, a form of limestone. Limestone is formed from the decomposition and sedimentation of Coccolithophores skeletons, a type of plankton.
Coccolithophores are chalk-forming plankton. They feature spherical skeletons called cocospheres, which are formed from calcareous discs called coccoliths. After death, the cocospheres and coccoliths settle down to the sea floor and decompose to their constituent parts. Over the years, the skeletons accumulate and form lime mud under the right conditions.
After geologic time has passed and sediments have started to build on top, sea water subsides, and the lime mud is subjected to tremendous heat and pressure. This removes the water and compacts the sediment layer into rock.
In order to manufacture slender sticks of chalk, limestone is first quarried. Next, the limestone is crushed by a jaw crusher. It is then wet-milled with water in a rotating steel drum with metal balls to crush the chalk further and wash away impurities. The chalk is then sifted over vibrating screens to separate large particles from the fine dust. It is then mixed with water, formed and cut to its proper length, and cured in an oven for four days.