A pearl is formed when some sort of small object, typically a parasite or piece of organic matter, becomes embedded in the tissue of an oyster or mollusk. To reduce irritation, the mollusk coats the intruder with the same secretion it uses for shell-building, nacre. As the nacre builds up in layers, it surrounds the irritant and eventually forms a pearl. It is a myth that a grain of sand can cause a pearl to form as nacre will not adhere to inorganic substances.
Natural pearls are those pearls which are formed in nature, more or less by chance. Cultured pearls, by contrast, are those in which humans take a helping hand. By actually inserting a foreign object into the tissue of an oyster or mollusk, pearl farmers can induce the creation of a pearl. The same natural process of pearl creation takes place.
Sources: Wikipedia, Britannica Online Encyclopedia
Oysters shells
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