Calcium carbonate is a chemical compound, with the chemical formula CaCO3. The elements involved are calcium (Ca), carbon (C) and oxygen (O). It is a common substance found as rock in all parts of the world, and is the main component of seashells, snails, and eggshell.
Calcium carbonate is usually the main cause of hard water. It is used as a calcium supplement or as a treatment for antacid, but too much can be bad for your health.