calcium carbonate

Calcium carbonate is a chemical compound, with the chemical formula CaCO3. It is a common substance found as rock in all parts of the world, and is the main component of shells of marine organisms, snails, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is usually the principal cause of hard water. It is commonly used medicinally as a calcium supplement or as an antacid.
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A mineral taken primarily as a supplement to prevent osteoporosis. It is also being studied for cancer prevention. 

Noun
1. a salt found in nature as chalk or calcite or aragonite or limestone
(hypernym) carbonate
(hyponym) dripstone
(substance-holonym) chalk