The hard, rigid outer calcium carbonate covering of certain animals is called a shell. While many animals, particularly those that live in the sea, produce exoskeletons, usually only those of mollusks are considered to be shells. It is sometimes said that shells are made of chitin, but these are unrelated materials (except for their hardness and use as a covering by animals).
The shell is usually made of nacre, an organic mixture of outer layers of horny conchiolin (a scleroprotein), followed by an intermediate layer of calcite or aragonite, and then a layer of calcium carbonate (CaCO3) in the form of platy crystals.
Nacre is secreted by the ectodermic cells of the mantle tissue of certain species of mollusk. Mollusk blood is rich in a liquid form of calcium. In these mollusks the calcium is concentrated out from the blood where it can crystallize as calcium carbonate. The individual crystals of each layer differ in shape and orientation. Nacre is continually deposited onto the inner surface of the animal's shell (the iridescent nacreous layer, also known as mother of pearl), both as a means to smoothen the shell itself and as a defense against parasitic organisms and damaging detritus.
When a mollusk is invaded by a parasite or is irritated by a foreign object that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of nacre. This process eventually forms what we call pearls and continues for as long as the mollusk lives.
Shells are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time. Large amounts of shells may form sediment and become compressed into limestone. Shells that wash up on beaches are called seashells, and are collected by some enthusiasts.