Natural Pearls & Cutltured Pearls
When an irritant 'accidentally' enters a wild oyster living in lakes, rivers and seas, the oyster then covers it with layer upon layer of nacre, and this eventually becomes a natural pearl. 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 and collected in most pearl jewelry stores and worth a lot more than cultured pearls.
Cultured pearls are started by man and then finished by nature, i.e. they are formed after a human intentionally inserts a shell bead nucleus or a piece of oyster or mussel tissue in a mollusk. Instead of an irritant "accidentally" entering the oyster, a pearl farmer deliberately and purposely implants a piece of mother of pearl shell into the oyster. Just as a farmer plants a seed and waits for it to grow, the pearl farmer takes care of his oysters so that nature assists him in creating his crop of pearls. Pearls on today's market are mostly cultured.
While natural and cultured pearls are extremely similar, there are a number of ways to tell the difference between them with the right equipments. The key difference between natural and cultured pearls lies in the structure of nacre layer.
Although it is a little expensive, X-radiograph test is the most reliable way to distinguish between natural and cultured pearls. On an x-radiograph negative, cultured pearls usually show a clear separation between core and nacre. Plus, their core normally looks lighter than the nacre coating. X-rayed natural pearls tend to either look the same tone throughout or get darker in their center. A mantle tissue nucleus will look like a very dark, irregular-shaped void. An X-ray photo is also an excellent source of additional information on the identification of natural pearls.
Also, Drill Hole Test is a simple optical way to identify the difference. Look inside the drill hole with a 10-power magnifier. If you can see a dark dividing line separating the nacre from a pearl bead nucleus, the pearl is cultured. This dark line is conchiolin, the material which binds the nacre to the bead. Natural pearls may show a series of growth lines, which get more yellow or brown towards the center of the pearl. A black deposit at the center of a white pearl can be a sure sign the pearl is natural. Also note the size of the drill hole. The drill holes of natural pearls are rarely larger than .04 mm (.016 inch). Those of cultured pearls tend to measure .06 mm (.024 inch). Natural pearls are partly valued by weight, so the holes are made as small as possible to minimize weight loss.
Simply, 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.