To identify natural, cultured, and imitation pearls, gemologists use tests and equipment ranging from the simplest to the most sophisticated.
The tooth test:
One of the fastest ways to distinguish between imitation pearls and natural or cultured pearls is the tooth test. Rub the beads lightly across the cutting edge of your teeth. The surfaces of natural and cultured pearls both have a slightly gritty feel. Imitations feel smooth or slippery on the teeth.
Blinking and candling test:
Separating natural from cultured pearls is now less a problem than it was in the early 1900s when cultured pearls were introduced. Today most pearls on the market are cultured. Natural pearls, both rare and expensive, occasionally crop up (usually as seed pearls in antique jewelry).
Two techniques, blinking and candling, can reveal the shell nucleus of a bead-nucleated cultured pearl. (It’s almost impossible, however, to prove natural origin without X-radiography).
With blinking, you rotate the strand under intense light. As you do this, you look for a flash at the point of rotation where the light passing through the nacre reflects back off the surface of the nucleus (perpendicular to the parallel layers in the shell bead).
With candling, you shine a narrow beam of intense light through the pearl from behind. At a certain point as you rotate the strand, the light passes between the parallel layers of the shell nucleus.
Blinking and candling can prove a strand is cultured. But the absence of a blink does not prove it is natural. If the nacre is very thick, or if the pearl formed around a tiny piece of implanted mantle tissue, there will be no blinking.
Magnification with 10X lens
By examining under magnification the area around the drill hole of a pearl, you can get a few more clues about its origin (cultured, natural, or imitation) Imitation pearls are frequently strung without knots between them. So the coating around the drill hole chips and flakes, revealing the glass bead underneath. Cultured pearls have a different look. Before the mollusk coats the nucleus with nacre, it lays down a layer of dark, non-lustrous material called conchiolin. If you shine light down the drill hole and look into it, you can often see this layer is not always visible. So you must look at many pearls on the strand.
X-radiography proves a strand of pearls to be natural or cultured. As X-rays pass through a cultured pearl, the conchiolin passes them, while the bead nucleus and nacre both transmit them less easily. Thus the tester can see the separation between the nucleus and nacre. Also, the nucleus is slightly more opaque to the X-rays than the nacre and gives a lighter image on the X-radiograph.
X-ray fluorescence by itself may give a strong indication (but not positive proof) that a strand is cultured. Used in conjunction with X-radiography, visible fluorescence to W-rays gives important information about whether the pearl is cultured or natural. The presence of fluorescence and its strength lets us distinguish between saltwater and freshwater pearls. Natural saltwater pearls do not fluoresce to X-rays at all. Because saltwater cultured pearls are usually nucleated with a freshwater mollusk bead, most of them fluoresce through their relatively thin nacre. If the nacre is very thick, as with the South Sea cultured pearl, they may fluoresce weakly, or not at all.