Seashells are perhaps the most familiar and most common shells, both in the wild and for sale as decorative objects. They are usually composed of calcium carbonate, in the form of calcite or aragonite. This can have different forms, one is nacre or mother of pearl. Other kinds of animal shells are made from chitin, bone and cartilage, or silica.
Use of Shells
Small pieces of colored shell and iridescent shell have been used to create mosaics and inlays which have been used to decorate larger items such as boxes and furniture. Large numbers of whole seashells, arranged to form patterns, have been used to decorate man-made grottos.
Several kinds of sea animals have exoskeletons which one can find after their death in beach drift. Often beachcombers pick them. But usually only those of molluscs (also spelled "mollusks") are known as seashells.
Shell (zoology) Facts for Kids. Kiddle Encyclopedia.