Limestone is a very common sedimentary rock. Most marine limestone is formed from the mineral calcite which comes from the beds of evaporated seas and from sea animal shells. The calcite is derived mostly from the remains of organisms such as clams, snails, oysters and corals. These animals live on the bottom of the sea and when they die their shells accumulate into piles of shelly debris. This debris can then form beds of limestone. Great sources of limestone are reefs. Some limestone are formed in fresh-water environments associated with caves and springs or lakes. Bands of limestone can emerge from the Earth's sruface in spectacular rocky outcrops and islands. Limestone is usually white but may be colored by iron oxide making it brown, yellow, or red; and carbon making it blue, black or gray. Limestone has been used for centuries. The Great Pyramid in Giza, Egypt is made of limestone. Today limestone is used for tile, flooring and countertops.

Photos: