Calcium carbonate

Calcium carbonate is a naturally occurring chemical compound. In medicine, it can be used both as a calcium supplement and as an active ingredient in an antacid. However, if taken in excess, it can lead to milk-alkalai syndrome, a potentially fatal condition.
Calcium carbonate is also the major component of limestone and the majority component of many ocean life forms, including pearls. It also forms limescale and, ironically, is usually the most common abrasive in household cleaners used to remove limescale. 
Calcium carbonate at Wikipedia