PEARL

Pearls form in the shells of freshwater and marine luscs. In theory, all types of these mollusks that have a shell can produce pearls. At the same time, only pearls covered with a layer of mother-of-pearl are of commercial value and they are created only by bivalves and some gastropods, as well as one type of cephalopods.
Pearls are formed as a result of a mollusk’s reaction to irritation when a foreign body enters the space between the shell valve and the mantle or to its introduction directly into the mantle. The latter is a fold of the body wall, covered with epidermis and forming a mantle cavity that communicates with the external environment. In bivalve mollusks, it has the form of two folds hanging from the back on the sides to the ventral side. The outer layer of the mantle contains a large number of glandular cells that produce various layers of the mollusk shell.
Pearls are the only jewelry material that forms in the body of mollusks, and they are probably one of the oldest used as jewelry, since they do not need additional processing. For centuries, high quality pearls have been sold for very large sums of money. Rare pearls are sold at auctions.