Limestone is widely found all over the world. Compared to most types of rock it is light in color (usually white, beige, or light grey), relatively soft, and dissolves slightly in water. Chalk is a type of limestone.
Unlike most types of rock, limestone is made of the remains of living creatures. Limestone is mostly calcium carbonate (CaCO3) derived from the shells of snails and shellfish and the exoskeletons of corals and other sea creatures that lived long ago.
Limestone can be used to make: lime mortar and lime plaster (traditional building materials), whitewash (a traditional paint and sealant), and agricultural lime (a soil conditioner / fertilizer).
In regions where there is plenty of limestone, there is likely to be dissolved calcium carbonate in the water supply. This hard water is harmless but it causes limescale, a crusty build-up of calcium carbonate deposits on taps, toilets and other bathroom fitings, and inside kettles, which is troublesome to clean.