Real Dictionary

limestone
Limestone is a whitish-coloured rock which is used for building and for making cement.

limestone in American English
a sedimentary rock consisting mainly of calcium carbonate, often composed of the organic remains of sea animals, as mollusks, corals, etc., and used as building stone, a source of lime, etc.: when crystallized by heat and pressure it becomes marble
noun: a sedimentary rock consisting predominantly of calcium carbonate, varieties of which are formed from the skeletons of marine microorganisms and coral: used as a building stone and in the manufacture of lime

limestone in British English
noun: a sedimentary rock consisting mainly of calcium carbonate, deposited as the calcareous remains of marine animals or chemically precipitated from the sea: used as a building stone and in the manufacture of cement, lime, etc