Cultured pearl is calcium carbonate and a fibrous protein called conchiolin. As the nacre builds up in layers of minute aragonite tablets, it fills the growing pearl sac and eventually forms a pearl. Natural pearls are formed by nature, more or less by chance. On the other hand, cultured pearls are human creations formed by inserting a tissue graft from a donor oyster, upon which a pearl sac forms, and the inner side precipitates calcium carbonate, in the form of nacre or "mother-of-pearl". The most popular and effective method for creating cultured pearls are made from the shells of freshwater river mussels
Pearl of Kuwait at the Victoria and Albert Museum in London, which closed on 19 January 2014. Its owners have made claims that it is the same pearl that is depicted in the portraits of Mary Tudor. The Pearl of Kuwait outweighs La Peregrina by a small amount. The pearl is currently on loan to the Smithsonian, who make no claims of its history or provenance. The Smithsonian does say that it is exceptional, that is weighs 64.4 carats, that its setting is 19th century in style, and that the pearl itself "recalls" those that originate from the Persian Gulf. Pearl of Kuwait