Pearls are round, hard masses formed in the shells of most living mollusks. A definition of a mollusk is any invertebrate of the phylum, typically having a calcareous shell. An oyster is a bivalve (meaning two shells) and belongs to a group of animals with shells called Molluscs.
Natural pearls form when an irritant - usually a parasite and not the proverbial grain of sand - works its way into an oyster. If the parasite embeds itself into the mollusk’s soft mantle, the mollusk will cover the parasite with a sac and begin to form layers of nacre around the irritant. Natural pearls are formed by nature, more or less by chance.
Nacre is the iridescent material that the mollusk forms in very thin layers that creates the pearl.Nacre is a smooth, shiny substance that makes up the interior of the mollusk inside an oyster. Over time, the layers formed around the irritant, will form a pearl. The thicker the nacre, the more durable and desirable the pearl, which means that time needed for thicker nacre is also a factor.
Cultured pearls are also created in mollusks, but they are formed by mollusks in captivity and have had irritants manually inserted. 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". Now a days a sand grain is inserted inside an oysters body by modern technology. As they grow naturally in an oyster they can not be considered as an artificial one.
Natural freshwater pearls come from mussels that live in ponds, rivers, and lakes. Pearls can also be cultured in freshwater, with the majority started by manually inserting a piece of the mussel’s own mantle into its flesh.
These are types of freshwater pearls:
Found primarily in the Red Sea, the Persian Gulf, and the coastal waters of India and Japan, naturally formed saltwater pearls closely resemble freshwater pearls in shape and color. They are produced by a saltwater mollusk, such as the pearl oyster. . Cultured saltwater pearls show a slight difference from cultured freshwater pearls, and are thought to be closer to a perfect sphere.
These are types of saltwater pearls:
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