Q: When I looked closely at the hole drilled into a pearl I’d bought, it looked like a thin shell of pearl over a bead. Is this normal, or was I ripped off?
A: You did not get ripped off. Completely natural, round white saltwater pearls are and have been almost unavailable since before World War II, when pollution in the Persian Gulf pretty much eliminated the traditional pearl fisheries there. Nowadays, what we usually call pearls are more precisely called cultured pearls. These involve inducing an oyster or other mollusk to produce a pearl, usually around an artificial nucleus. The best known of this type are the Japanese Akoya cultured pearl: the traditional-looking white round pearl, often with pinkish coloration in the best qualities, other times white, cream colored, yellowish, etc, with a slightly iridescent surface. These are now what you think of as a pearl.
The most commonly used nucleus for these pearls is a carefully cut round bead, often made from Mississippi River clam shells. These are surgically implanted into the oyster, and left there anywhere from six months to a couple of years. The thickness and quality of the pearl layer deposited on the bead depends on the time, water temperature, health of the animal, etc. Pearls with thin or chalky looking outer layers (called the nacre of the pearl) are really cheap, and easily can have that outer layer chipped off or worn through. Better pearls with thicker nacre layers are usually more durable, and are worth more.
Pearls can easily get damaged, however. When a section of the pearl chips away, often near the drill hole due to stress in drilling, or the stress of being pulled off, you then will see the smooth, often glassy-looking surface of the inner clamshell bead. Sometimes, for some designs, a so-called ¾ pearl is used, as they are slightly less expensive. In these, a flat surface is ground into the pearl (often to remove a blemish), with the drill hole in the middle. These can fit a bit closer into a design, or tighter to a flat surface, and the flat ground spot is usually hidden until the pearl comes off. Then you can clearly see the structure of the pearl, with the outer nacre surrounding the core bead.
There are also imitations made. Several involve coating a glass or plastic bead with an iridescent paint or lacquer. Most of the time, these are easily distinguished from the real thing, as on close inspection the surface lacks the proper iridescence, or translucency, or feel of a true cultured pearl or a true natural pearl (which has the same outside look, and only looks different inside, or in an x-ray.) Also, the coating on these is usually extremely thin, like thin paint. Even the poorest quality of cultured pearl usually has a nacre that’s around .2 mm thick or better, about like a piece of paper’s thickness. And the better ones will be closer to a .5 mm or more, which starts to look like a distinctly thick shell, about like the thickness of your fingernails.