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. Calcium carbonate is widely used medicinally as an inexpensive dietary calcium supplement or gastric antacid. It is also used in the pharmaceutical industry as an inert filler for tablets and other pharmaceuticals. Calcium carbonate is used in the production of calcium oxide as well as toothpaste and has seen resurgence as a food preservative and colour retainer, when used in or with products such as organic apples or food. CaCO3 can be added to neutralize the effects of acid rain in river ecosystems. Currently calcium carbonate is used to neutralize acidic conditions in both soil and water.