Limestone is a sedimentary rock containing the mineral calcium carbonate or calcite. Limestone includes decorative rock types as well as chalk, tufa and coral.

Formation
Limestone, like all sedimentary rock, forms when layers of eroded mineral and organic material settle on the earth's crust and build up over eons. Limestone was formed from ancient marine animals that sank to the bottom of shallow seas when they died. It continues to form today in shallow seas, the best-known growing deposit being near Bermuda in the Atlantic Ocean.

Uses
Many 19th and early 20th Century buildings were constructed of limestone.Limestone is used in road-building, construction, landscaping and cement production. Because of its durability, limestone has been popular as a building material since the late 19th Century. Many historic buildings, including banks, government offices and train stations from the late 19th and early 20th centuries were made of limestone.

Considerations
Limestone's weight makes it inappropriate for use on tall buildings other than as a veneer. While it resists erosion and damage from wind and precipitation, limestone is susceptible to damage from highly acidic rain, which can slowly wear away details in building facades and monuments. Naturally occurring underground acid has eaten through limestone deposits to create great caves such as Luray Caverns in Virginia and Carlsbad Caverns in New Mexico.

Geology
Most limestone developed in ancient shallow seas. Deposits of marine animal skeletons provide the calcium that created limestone deposits. Layer after layer built up and compressed into rock under extreme pressure. Limestone deposits can be tens of feet thick or more and are usually removed from the ground in large quarries.

Variations
Limestone must include at least 50 percent calcite. Limestone deposits generally create a white rock, but impurities may add color. Iron oxide creates limestone with brown, red or yellow coloring while carbon adds blue, black or gray tones to limestone.

Other Uses
A variety of non-construction industries use limestone, including as filler in paper products, paint, rubber and plastic. Coal-burning power plants and sewage treatment plants use limestone as a filter to absorb pollutants