Limestone, chalk and marble are different forms of-

Limestone, chalk, and marble are all different forms of calcium carbonate (CaCO3). These materials have the same chemical composition but differ in their crystalline structure and formation process. Limestone is a sedimentary rock formed from the accumulation of calcium carbonate shells and skeletons of marine organisms. Chalk is a soft, white, porous rock made up of the microscopic remains of marine plankton.