The “pearl” is literally formed by the mantle epithelium, which becomes deposit around any external body or particles, these external particle or body invade in between the shell and mantle in any way.
Pearl is secreted by the mantle as a means of protection against a small external particle. When an external particle or body, such as a grain of sand or a small parasite invades in between the mantle and the shell it becomes enclosed in a sac of mantle epithelium which produces irritation. The irritation stimulates the mantle epithelium to secrete thin concentric layers of mother of pearl around the foreign body. The amount of deposition is in direct proportion to the degree of irritation. After several years, a pearl will be formed, usually, it requires 3 to 4 years to produce a pearl of considerable size but a large pearl requires about 7 years. The foreign particles in the pearl are called nucleus whereas the thin nacre layers are concentric and called the mother of pearl.
Pearls are formed by various clams and oysters, but those produced by the marine pearl oysters, Meleagrina of Eastern Asia are the most valuable.