Limestone is a white sediment rock made predominantly of calcium carbonate. Depending on its origin, limestone can be either hemogenic (chemically precipitated), biogenic (made predominantly of remains of organisms – phyto- and zooplankton, shells, skeletal particles, etc.) or clastic (made of broken pieces of older limestone). There are a few types of limestone: ordinary, shelly, and chalky. Ordinary limestone is used as a building material and to make lime and cement. Shelly limestone gets its name from the shells of different animals that can be seen in it. It is quite beautiful and is used in the linings of buildings, water fountains, etc. The chalky variety is very soft (soft limestone). It is what blackboard chalk is made of.
Limestone is one of the rocks most commonly used in the building industry.