The difference between cultured and natural pearls is very simple. With cultured pearls, the oyster/mollusk is given a "helping hand" by man. In the formation of both natural pearls and cultured pearls the oyster or other mollusk receives an irritant or intruder inside its shell. As a result it secretes a coating called nacre to cover the irritant. In the case of Natural Pearls this is done naturally by an irritant or intruder which is likely a parasite or foreign object. The result is the development of a pearl. Since these pearls occur entirely by accident in the wild, some people call them wild pearls.
For cultured pearls, a pearl farmer surgically places an irritant in the form of mantle tissue and/or a small bead, often a piece of shell from another mollusk into the oyster/mollusk. Originally developed by Mikimoto and used for Japanese Akoya pearls, this system has now been used in other type of molluscs to produced cultured pearls of other varieties. For both Natural or Cultured Pearls everything else is the same up until it is time to remove the pearl. This is Mother Natures gift!
Natural pearls are very rare today. Most of the areas where natural pearls could be found a couple hundred years ago no longer have enough oysters to make searching for the odd pearl there commercially viable. Such pearls are rarely round in shape, nor very large. So most of the pearls found on the market today have come from oysters/mollusks that have been protected by man and induced to produce their pearls. These are known as cultured pearls. They may be Akoya pearls, Freshwater pearls, Tahitian pearls or South Sea pearls. All of these are classified as cultured pearls, because of man's role in stimulating the molluscs to produce a pearl.
Examples of natural pearls can include pearls from the Persian Gulf, conch pearls, abalone pearls, melo pearls, quahog pearls, blue mussel pearls, etc.