Limestone is a very popular sedimentary rock of biochemical origin. It is composed primarily of the mineral calcite. Occasionally it is almost pure calcite, but most limestone is filled with lots of additional minerals and sand which result in the term dirty limestone. The calcite is obtained mainly from the remains of organisms such as clams, brachiopods, bryozoan, crinoids and corals. Limestone forms generally close to the source of shelly debris although some significant transport can occur.