A nucleus is introduced by pearl farmers for Pearl Formation. It is popularly thought that pearls are formed when a grain of sand is introduced in the oyster and it is coated as a natural defense, and this is so, but the probability of finding an oyster in the ocean, which has formed a quality and round pearl is of 1 in 1 million. That is why pearl growers introduce a nucleus which is normally mantle tissue, so that the pearl surrounds it and segregates nacre. Once this process is done, it will take 2-3 years for the oyster to form a pearl.
All this process is observed by expert cultivators, who watch a multitude of factors that can affect the formation of the pearl, such as the temperature of the water, currents, that the water is not clean or inclemencies of the time. South Sea Pearls are the type of pearl that takes by far the longest time to form.