calcium carbonate

:  a solid substance that is found as limestone and marble and in plant ashes, bones, and shells

:  a calcium salt CaCO₃ that is found in limestone, chalk, marble, plant ashes, bones, and many shells, that is obtained also as a white precipitate by passing carbon dioxide into a suspension of calcium hydroxide in water, and that is used in dentifrices and in pharmaceuticals as an antacid and to supplement bodily calcium stores