Natural and cultured pearls both grow inside oysters. However, natural pearls form naturally in oysters while cultured pearls are formed in oysters by the human implantation of a nucleus. The other main difference between the two is that a natural pearl has thicker mother of pearl, or nacre, layers than a cultured pearl.
The nacre layers formed on all pearls are made up of microscopic crystals. The iridescent finish on pearls comes from the perfect alignment of the crystals so that they reflect light. Nacre is 90% calcium carbonate and 10% water and other organic substances. The outer layer of can look very different between natural and cultured pearls; natural pearls are often less iridescent than cultured ones.
Natural and cultured pearls can also look extremely different inside. Sometimes, a strong light is enough to see inside a pearl, but usually a pearl's inner layers can only be seen with special x-ray-like equipment. Cultured pearls often show a narrow brown line around the nucleus, while natural pearls have none of these growth rings.
The Japanese invented the process of the nucleus method in pearl harvesting. The nucleus implanted into the oyster by humans is usually larger than a natural pearl would be since the nacre layers will not be as thick in a cultured pearl. Otherwise, the oyster coats both with nacre as a means of protecting itself from the foreign irritant.
Contrary to popular thought, a grain of sand is not usually enough of an irritant to an oyster to cause it to form a pearl. Natural pearls are often formed from a fish scale, a parasite or a piece of shell. Both natural and cultured pearls are in demand in today's market, but most pearls sold today are cultured. Unless pearls are more than 80 years old, it is a good chance that they are cultured rather than natural.
Natural pearls are rare and most mollusks capable of producing pearls are nearly extinct due to pearl hunting in the 1800s. Bahrain is one of the few remaining countries that has pearl divers looking for natural pearls. Any type of pearl can be expensive as not every oyster makes them. Only one in every 10,000 oysters will produce a pearl, and only a few will have the desired luster and shape.
The appearance of both natural and cultured pearls depend not only on the irritant, but on the water and food conditions where the oyster is living. Some Chinese pearl farmers grow cultured freshwater pearls that are very thick in nacre because they move the pearl nucleus as it grows in the oyster so that the resulting shape will be more evenly rounded. Akoya pearls are cultured pearls produced by little Japanese oysters. Rounded beads are implanted into the oysters so that Akoya pearls are usually much rounder than most cultured freshwater pearls.