Limestone is a white sedimentary rock, composed primarily of calcium carbonate. Most widely it is used in construction in its natural form, but also in industrial production. Like most sedimentary rocks, limestone is composed of grains which are skeletal fragments of marine organisms such as coral or foraminifera. Limestone may be crystalline, clastic, granular or massive, depending on the method of formation. Many sites and monuments around the world are made of limestone, including the Great Pyramid of Giza. An interesting fact is that a great number of buildings in Kingston, Canada were built of limestone, so it is called 'Limestone City'. Limestone is very beautiful and improves with age.