Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO₃).

Limestone has numerous uses: as a building material, an essential component of concrete (Portland cement), as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.

Description：Like most other sedimentary rocks, most limestone is composed ofgrains. Most grains in limestone are skeletal fragments of marine organismssuch as coral or foraminifera. These organisms secrete shells made of aragonite or calcite,and leave these shells behind when they die. Other carbonate grains comprisinglimestones are ooids, peloids, intraclasts, and extraclasts.

Uses：Limestone is very common in architecture, especially in Europeand North America. Many landmarks across the world, including the Great Pyramid andits associated complex in Giza, Egypt, were made oflimestone. So many buildings in Kingston, Ontario, Canada were, and continue to be, constructed from it that it isnicknamed the 'Limestone City' On the island of Malta, a variety of limestone called Globigerina limestone was, for along time, the only building material available, and is still very frequentlyused on all types of buildings and sculptures. Limestone is readily availableand relatively easy to cut into blocks or more elaborate carving. It is also long-lasting and stands up well to exposure.However, it is a very heavy material, making it impractical for tall buildings,and relatively expensive as a building material.