Natural pearls are grown in wild oysters. The process begins when an irritant, such as a grain of sand or a piece of shell, gets inside an oyster's shell. To protect its delicate body from the irritant, the pearl secretes a substance called nacre to surround the irritant. The oyster never stops secreting nacre, so over the years the nacre layers build up into a genuine pearl.
While natural and cultured pearls are extremely similar, you can tell the difference between them with the right equipment. The key difference between natural and cultured pearls lies in the nacre layer. A natural pearl's nacre layer is much thicker than that of a cultured pearl, even when the two have a similar diameter.
Natural pearls are less transparent than cultured pearls. If you place a natural pearl against a dark background—like a box lined in black cloth—and put it under a strong light, the natural pearl will look like a small, white, homogenous ball with no discernible inner rings. When you do the same to a cultured pearl, you will see a thin brown line between the nacre layer and the nucleus of the pearl.
Under an X-ray, natural and cultured pearls have a different internal structure. Cultured pearls will show concentric growth rings, while natural pearls will appear internally solid, with no growth rings. This is because natural pearls are 100% nacre, whereas cultured pearls have an inner nucleus composed of the bead that was deliberately injected to produce a pearl. That is why natural pearls are sold by weight (carat of pearls) while cultured pearls are sold by size (diameter of pearls).
Natural pearls are extremely rare. In nature, only about one in 10,000 oysters will produce a pearl. Of those, only a small percentage will achieve the size, shape, and color of a desirable gemstone. The odds of finding a perfect natural pearl are around one in a million. Because of their rarity, natural pearls are hardly ever found in most pearl jewelry stores. Most pearls for sale in the market today have been cultured.