Limestone is a sedimentary rock composed primarily of calcite, a calcium carbonate mineral with a chemical composition of CaCO3. It usually forms in clear, calm, warm, shallow marine waters. It is usually a biological sedimentary rock, forming from the accumulation of shell, coral, algal, fecal, and other organic debris. It can also form by chemical sedimentary processes, such as the precipitation of calcium carbonate from lake or ocean water.