Natural pearls, are pearls formed by chance. Cultured pearls have been given a helping hand by man. Today, nearly all pearls are cultured. By inserting a foreign object into a mollusk, pearl farmers can induce the creation of a pearl. From there, the same process of natural pearl creation takes place. The difference is that in this case, the inducement is intentional.
Natural pearls are formed randomly and really are simple accidents of nature. When a certain type of irritant, such as a parasite, becomes lodged in the tissue of a mollusk, the animal responds by secreting a calcium carbonate substance called nacre to coat the intruder and protect the mollusk. Over a period of several years, this build-up of nacre forms a natural pearl. Where as Cultured pearls are still real pearls, grown organically inside of oysters in the same way as natural pearls. The difference is, that in the case of cultured pearls, the pearl farmer intentionally stimulates the development of the pearl by inserting a "nucleus" into the oyster. Thus, the formation and discovery of the pearl are no longer left to chance.
cultured pearls tend to be perfectly spherical in shape, while natural pearls may or may not be spherical. Similarly, you will see a slightly greenish tinge in cultured pearls which won't be seen in natural pearls. Cultured pearls can be distinguished from natural pearls through the use of x-rays, which reveal the inner part of the pearl. Any pearl that is grown and Pearl Jewellery marketed today, are considered cultured pearls. This is because these pearls would not exist without human intervention. A freshwater pearl, although it may be solid nacre, is a cultured pearl where the mussel was nucleated with a piece of mantle tissue in their mantle tissue. All pearls cultured in saltwater are nucleated in the gonad with a piece of mantle tissue, and a mother-of-pearl bead, which is how akoya pearls are nucleated. This is why akoya pearls are cultured pearls, but they are saltwater, not freshwater, and much different in appearance and more valuable than freshwater pearls.