Like other gemstones, pearl is not produced in the deep down under earth. Rather it is produced by a living organism.
Pearls are produced within the soft tissues of a living mollusk better known as oysters and mussels. Mollusks generally have an unsegmented body and a hard exterior shell, like a snail. They are found in marine and freshwater. Any mollusk that produces a shell can produce a pearl.
Pearl is not just formed on their own in the mollusks. They are formed when some kind of small object, typically a parasite or a piece of organic matter, becomes embedded in the tissue of an oyster or mollusk. As a result, the mantle tissue of the mollusk secretes nacre, chemically known as calcium carbonate or conchiolin. As the nacre builds up in layers, it surrounds the irritant and eventually forms a pearl. These pearls are known as natural pearls.