What is Lime Stone: Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3).  Like most other sedimentary rocks, limestones are composed of grains; however, most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts. Some limestones do not consist of grains at all and are formed completely by the chemical precipitation of calcite or aragonite. i.e. travertine.