What Is Limestone Rock?

Formation

• Most limestone is formed when bits of rock, coral, shell and skeletal remains of minute sea creatures fall together to form huge piles of sediment in the ocean. These are compressed and cemented together over thousands of years, building up in layers over time. In caves, evaporation can form limestone stalactites and stalagmites. Drops of water evaporate and leave behind a calcium carbonate deposit on the floor or ceiling of the cave, accumulating into an icicle shape over thousands of years.

Picturesque Examples

• Limestone sometimes emerges from the earth's surface to form rocky outcrops or islands. These can be fissured and picturesque, such as the Verdon Gorge in France, Malham Cove in England and Ha Long Bay National Park in Vietnam. Though it is white in its pure form, other substances such as clay found in the limestone can create an array of different colors, including gray, black, yellow, and, most rarely, red.

Classification

• In order for a rock to be classified a limestone, it must contain at least 50 percent calcite, according to the National Audubon Society Field Guide to Rocks and Minerals of North America. The remainder contains grains of other substances, including quartz, clay, rock particles or iron oxide. Though it is an extremely heavy material, limestone is easily weathered, often crumbly, pitted or pockmarked, and is soluble in hydrochloric acid.

Uses

• Limestone has many uses. It is an ingredient in cement, mortar, iron and steel production, concrete, cleaning products and glass. It is made into fireplaces and can be honed for floor tile. In the late 19th and early 20th centuries, many public buildings such as train stations, banks and municipal buildings were constructed of limestone blocks. Some of them are decorated with relief carvings, but the details are susceptible to damage from acid rain.

Varieties

• Limestone comes in many varieties. The least common is classic limestone, which is composed of broken shell fragments and calcite crystals and washes up on beaches. Oolitic limestone is found in tropical warm, shallow marine water. Travertine, another type of limestone, is deposited in caves and has bands of color. Chalk is a pure white, porous, soft type of limestone also found in shallow sea water.