In one of its most natural forms, chalk is composed from the accretion and sedimentation of trillions of skeletons of long-dead micro-organisms.
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate.