Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the mineralscalcite and aragonite (most notably as limestone) and is the main component of pearls and the shells of marine organisms,snails, and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react withcarbonate ions to create limescale. It is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardousPreparation[edit] The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (e.g. for food or pharmaceutical use), can be produced from a pure quarried source (usually marble).Alternatively, calcium carbonate is prepared from calcium oxide. Water is added to give calcium hydroxide then carbon dioxide is passed through this solution to precipitate the desired calcium carbonate, referred to in the industry as precipitated calcium carbonate (PCC):[7] CaO + H2O Ca(OH)2Ca(OH)2 + CO2 CaCO3 + H2O