Calcium carbonate is a common substance found in rocks in all parts of the world, mainly as the minerals calcite and aragonite. Calcium carbonate is the active ingredient in agricultural lime, and its presence can usually one of the causes of hard water. Food-grade calcium carbonate is usually quarried from a pure source, such as marble. It is used in some pharmaceuticals as a calcium supplement or as an antacid, but excessive consumption can be hazardous. When used in toothpastes, it can function as a mild abrasive to gently clean teeth. It is also the main component of the shells of marine organisms, snails, coal balls, pearls, and eggshells.