Limestone is a sedimentary rock composed almost entirely of the mineral calcite (calcium carbonate, CaCO₃). The precursor calcium-carbonate sediment that existed prior to lithification of limestone can be of several types. These sediment types include carbonate mud, carbonate fossil fragments, carbonate pellets and rip-up clasts, and ooids. Carbonate mud is made of microcrystalline calcite crystals (crystals of a few microns in size) that form directly from seawater and from the disintegration of some calcareous marine algae. Carbonate fossil fragments include all shelly organic debris originally composed of CaCO₃ (in the form of calcite or a denser phase, aragonite). Carbonate pellets and rip-up clasts are small lumps of carbonate mud (a few millimeters in size) that have been consolidated either by being eaten and excreted (pellets), or by settling and then being ripped-up by wave energy (rip-up clasts). Ooids are sand-sized, concentrically layered grains that form by inorganic precipitation...