<P> Limestone has numerous uses: as a building material, an essential component of concrete (Portland cement), as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens . </P> <P> The first geologist to distinguish limestone from dolomite was Belsazar Hacquet in 1778 . </P> <P> Like most other sedimentary rocks, most limestone is composed of grains . Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera . These organisms secrete shells made of aragonite or calcite, and leave these shells behind when they die . Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts . </P> <P> Limestone often contains variable amounts of silica in the form of chert (chalcedony, flint, jasper, etc .) or siliceous skeletal fragment (sponge spicules, diatoms, radiolarians), and varying amounts of clay, silt and sand (terrestrial detritus) carried in by rivers . </P>

Where does the carbon in limestone come from