Calcium carbonate is a chemical compound with the formula CaCO₃. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone), and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is created when calcium ions in hard water react with carbonate ions creating limescale. It is commonly used medicinally as a calcium supplement or as an antacid, but excessive consumption can be hazardous.