Limestone is a sedimentary rock that is composed primarily of calcite (calcium carbonate, CaCO 3) and dolomite (calcium magnesium carbonate, CaMg(CO 3) 2). It can vary in purity, consistency and hardness. Limestone is a sedimentary rock composed mostly of the mineral calcite and comprising about 15% of the Earth’s sedimentary crust. Limestone – which is a sedimentary rock – is a valuable resource from the Earth’s crust. Sandstone is created from particles of other rocks and sand. Limestone definition, a sedimentary rock consisting predominantly of calcium carbonate, varieties of which are formed from the skeletons of marine microorganisms and coral: used as a building stone and in the manufacture of lime. It is a basic building block of the construction industry (dimension stone) and a chief material from which aggregate, cement, lime and building stone are made. It is composed mainly of calcium carbonate, which comes from the ancient remains of sea creatures, such as coral and shellfish. Some are physical characteristics while others are chemical in nature. Thanks to its abundance and high calcium content, limestone has seen use across the world and throughout history in a range of applications. See more. Limestone is a common substance, making up about 10% of sedimentary rocks across the world. 71% of all crushed stone produced in the U.S. is either limestone or dolomite. Limestone. Limestone is used in the construction industry because of its abundance and hard physical nature. It has many uses. All rocks have unique properties that set them apart.