Which compound is the main component of natural limestone?

Calcium carbonate is the main component of most natural limestone.

Its chemical formula is CaCO₃. Limestone is a sedimentary rock formed in many settings, commonly from accumulated shells and skeletons of marine organisms that contain calcium carbonate. It can also form through direct chemical precipitation from water.

Calcium carbonate occurs in several minerals and biological materials, including calcite, aragonite, chalk, marble, seashells, and much of eggshells. Limestone is not chemically identical in every deposit, because natural rock can contain clay, silica, dolomite, and other impurities.

Acids react with calcium carbonate and release carbon dioxide, which is why dilute acid produces fizz on limestone or marble. Calcium carbonate is also used to make cement, lime, paper coatings, and antacid products. Calcium sulfate, by contrast, is associated with gypsum rather than ordinary limestone.