Limestone is a sedimentary rock consisting of more than 50% calcium carbonate (calcite – CaCO3). There are many different types of limestone formed through a variety of processes. Limestone can be precipitated from water (non-clastic, chemical or inorganic limestone), secreted by marine organisms such as algae and coral (biochemical limestone), or can form from the shells of dead sea creatures (biolistic limestone). Some limestones form from the cementation of sand and / or mud by calcite (clastic limestone), and these often have the appearance of sandstone or mudstone. As calcite is the principle mineral component of limestone, it will fizz in dilute hydrochloric acid.