How are sea shells formed? eroded from what?

Answer 1

Sea shells are animals.
The hard, rigid outer 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.
The shell is usually made of nacre, an organic mixture of outer layers of horny conchiolin (a scleroprotein), followed by an intermediate layer of calcium carbonate (CaCO3) as either calcite or aragonite in the form of platy crystals.

Nacre is secreted by the epithelial cells (formed by the germ layer ectoderm) of the mantle tissue of certain species of mollusk. Mollusk blood is rich in dissolved 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.

The shell may grow over time as the animal inside adds nacre to the leading edge near the opening. This causes the shell to become longer and wider to better accommodate the growing animal inside.

http://en.wikipedia.org/wiki/Animal_shell