Black pearls are farmed in the warm South Pacific waters surrounding Tahiti and the Cook Islands. Naturally occurring black pearls are very rare, so they are cultivated in large quantities by pearl farmers. To cultivate a black pearl, pearl farmers insert a foreign object into an oyster shell to attract layers of mother-of-pearl around it. Then the oyster is left in the ocean for up to 18 months to allow the pearl to grow. The longer a pearl is left to grow inside the oyster shell the larger it will be, but the higher the risk of it becoming disfigured and worthless. Once the oysters are ready to be harvested they are collected from the ocean and the pearl is carefully removed. The oyster shell is then re-seeded and returned to the ocean so that it can be harvested again next season.