A faux pearl is generally understood to mean any sort of pearl that is made of something other than true nacre, be it plastic, glass, or resin. It is not usually understood to be a cultured pearl, even though this is not, strictly speaking, a natural pearl. Real or cultured pearls may also be treated, either with dyes or special coatings, and can come from a variety of mollusks in addition to oysters. There are a few different ways to distinguish a fake pearl from a real pearl, and even to distinguish a cultured pearl from a real pearl, some of which are more effective than others.
The most common way given to tell a faux pearl from a real pearl is usually known as the tooth test. The premise of the tooth test is that you rub the pearl along your teeth, and if the pearl feels smooth, it is a fake, whereas if it feels gritty it is a real pearl. This is a great sounding test, in theory, because anyone can do it at any time. The problem is, it is fairly unreliable, since a number of situations can cause a false-positive response, where you may think a faux pearl is a real pearl.
The idea behind the tooth test is simple, and at first glance seems like a good idea. A real pearl is made up of many different layers of nacre, built up around a central irritant, and so they are irregularly shaped and have bumps in them which can be detected against the tooth. A faux pearl, on the other hand, is usually machine-made of plastic or glass, and so will be perfectly smooth. However, there are plenty of exceptions to this.
For one thing, many a cultured pearl, although not actually a fake pearl, will have little enough nacre on it that it may feel smooth. Or it may have been dyed or treated, which will fill in the irregularities and also make it feel smooth. On the other hand, a faux pearl can be intentionally made with irregularities to make it feel more like a real pearl. So while the tooth test might work for distinguishing the most brash fake pearl from an untreated natural pearl, it would be less likely to discern the difference between a finely made faux pearl and a coated cultured pearl.
To really be certain about whether a pearl is fake or real, one usually needs to either destroy a pearl, or have access to more advanced equipment. With multiple pearls, one can be sacrificed and cut open to look at the layering inside. A real natural pearl will have countless layers of thin nacre, a cultured pearl will have a thick core of mother of pearl and a thin layer of nacre, and a fake pearl will have a core coated with a flaky substance. Without destroying the pearl, an x-ray can be used to get an even better look inside to determine all sorts of things about the pearl, including its authenticity.
One of the easiest ways to distinguish a faux pearl from a real pearl doesn’t even involve rubbing it against your teeth, but just looking at a group of pearls. If you place a collection of pearls, such as those in a necklace, under a bright light, the color and sheen of the pearls becomes much more obvious. Natural pearls will have slight variations in the coloring, and the shine may be more intense on some, while faux pearls will be almost identical in their coloring.