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.