limestone

Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). Limestone often contains variable amounts of silica in the form of chert or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock. The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze or alternatively is conglomerated in a coral reef (see lysocline for information on calcite dissolution). Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Another form taken by calcite is that of oolites (oolitic limestone) which can be recognized by its granular appearance. Limestone makes up about 10% of the total volume of all sedimentary rocks .
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(n.)
A rock consisting chiefly of calcium carbonate or carbonate of lime. It sometimes contains also magnesium carbonate, and is then called magnesian or dolomitic limestone. Crystalline limestone is called marble.
  

A sedimentary rock composed primarily of calcium carbonate. 10% to 15% of all sedimentary rocks are limestones. Limestone is usually organic, but it may also be inorganic.

Noun
1. a sedimentary rock consisting mainly of calcium that was deposited by the remains of marine animals
(hypernym) rock, stone
(hyponym) rottenstone, tripoli
(substance-meronym) calcium, Ca, atomic number 20