The formation of a natural pearl begins when a foreign substance slips into the oyster between the mantle and the shell, which irritates the mantle. It's kind of like the oyster getting a splinter. The oyster's natural reaction is to cover up that irritant to protect itself. The mantle covers the irritant with layers of the same nacre substance that is used to create the shell. This eventually forms a pearl.
Most pearls are nicely rounded objects, which are the most valuable ones. Not all pearls turn out so well. Some pearls form in an uneven shape -- these are called baroque pearls.
Cultured pearls are created by the same process as natural pearls, but are given a slight nudge by pearl harvesters. To create a cultured pearl, the harvester opens the oyster shell and cuts a small slit in the mantle tissue. Small irritants are then inserted under the mantle.
The natural color of a pearl results from a combination of several factors. The pearl's body color is its main color. This can be white, silver, cream-colored, gold, green, blue, or even black. The body color is determined by the type of oyster or mollusk that produces the pearl, as well as the conditions of the water, and sometimes the type of nucleus implanted to stimulate the pearl's creation.