calcium carbonate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A colorless or white crystalline compound, CaCO₃, occurring naturally as chalk, limestone, marble, and other forms and used in a wide variety of manufactured products including commercial chalk, Portland cement, medicines, and dentifrices.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun inorganic chemistry A colourless or white inorganic compound, CaCO3, occurring as chalk, limestone, marble etc; reacts with acids to liberate carbon dioxide.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a salt found in nature as chalk or calcite or aragonite or limestone