Sea shells are actually external skeletons which have kept their color and luster long after the death of the tenant. They are the hard coverings of a marine animal known as the mollusk. This casing, which protects its soft, defenseless body from predatory sea dwellers, is made up largely of carbonate of lime and is often stone-hard.
Shells usually have three layers. The outer covering of horny skin forms the protective surface. The middle and thicket layer is made up of prisms and governs the color pattern of the shell. Innermost is the smooth, shining nacreous lining often called mother-of-pearl. From this delicate surface, which gleams blue, rose, green and other pastel shades, is taken the material for pearl buttons, jewelry and adornments.