Seashells, which serve as the external skeletons of some sea animals, are made mostly of calcium, like other skeletons. Seashells that have been broken down into grains help make up the sand found on tropical beaches, according to Live Science.
Most of the seashells found on beaches are from mollusks, partially because these shells are very hard and tend to hold up better than other shells. Other seashells include barnacle shells, brachiopod shells, bivalve or clam shells, snail shells and crab shells. However, the term "seashell" is often used specifically to describe mollusk shells, especially in conchology, which is the study of seashells.
A type of seashell called a cowry shell has been used as money or a means of exchange in the past in India, China, North America and Africa. In North America, they were used by the Ojibway people.
While many seashells are collected by people who come across them naturally, many commercial seashells are harvested from living animals, a practice that can negatively affect ecosystems and harm rare species. The hermit crab is another creature that collects seashells, according to Pawnation. Hermit crabs are soft-bodied crustaceans that recycle discarded seashells by using them as shelters to protect themselves.