Limestone is a sedimentary rock largely composed of calcium carbonate (CaCO 3 ). Rocks with more than 50% calcium carbonate are considered to be limestone. Around 20 to 30 million years ago, when the climate was warmer and crust of the earth was mainly submerged under shallow seas, conditions were ideal for limestone formation. Coastal waters and layers of shells and bones from billions of sea creatures contributed to the formation of Limestone rocks. Limestone rocks, and in some cases marble, is important to the construction industry as a major building stone and aggregate.