Limestone is a sedimentary rock composed largely of the minerals calcite and/or aragonite, which are different crystal forms of calcium carbonate (CaCO3).

Like most other sedimentary rocks, Limestones are composed of grains; however, most grains in Limestone are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts. Some limestones do not consist of grains at all, and are formed completely by the chemical precipitation of calcite or aragonite, i.e. travertine.

The solubility of Limestone in water and weak acid solutions leads to karst landscapes. Regions overlying limestone bedrock tend to have fewer visible groundwater sources (ponds and streams), as surface water easily drains downward through joints in the Limestone. While draining, water and organic acid from the soil slowly (over thousands or millions of years) enlarges these cracks; dissolving the calcium carbonate and carrying it away in solution. Most cave systems are through Limestone bedrock.

For uses, Limestone is the raw material for the manufacture of quicklime (calcium oxide), slaked lime (calcium hydroxide), cement and mortar. Pulverized Limestone is used as a soil conditioner to neutralize acidic soils. It is crushed for use as aggregate—the solid base for many roads. Geological formations of limestone are among the best petroleum reservoirs; As a reagent in flue gas desulfurization, it reacts with sulfur dioxide for air pollution control. Glass making, in some circumstances, uses Limestone.
It is added to toothpaste, paper, plastics, paint, tiles, and other materials as both white pigment and a cheap filler. It can suppress methane explosions in underground coal mines.