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There are real pearls and then there are fake (faux) pearls...
If you are buying pearls, you may be able to identify fake pearls by their name: simulated, faux, glass, plastic, resin, artificial, manmade. Genuine pearls come in a variety of 'flavors'.
There are natural pearls and cultured pearls. Real pearls may come from freshwater or saltwater and from a variety of molluscs (not just oysters). To further confuse things, there are also shell pearls and genuine pearls, which have had treatments applied to them, such as coatings or faceting.
Before you buy or sell pearls, you need to know whether they are real or not. Here are some ways to tell them apart. The following are "tests" which can help you discern the quality of the pearls you have purchased.
Destruction Test (not recommended)
X-Ray
The Tooth Test
The Sun Test
Other Visual Clues
Density
Hole Characteristics
The Settings
Destruction:
If you cut a pearl open, you will see its true nature. Natural pearls are comprised of layer upon layer of nacre. Cultured pearls have a shell (mother-of-pearl) core covered with a thin layer of nacre (generally no more than half a millimeter, usually much thinner). Fake pearls have a core with one or more layers of coating applied to them, which tends to flake away from the core upon cutting. Cutting a pearl down the middle reveals the nature of its drill hole, if present. Of course, you need to be able to tell pearl-colored glass from shell in order to do this test (plastic and resin are easier to discern). Also, you'll destroy the pearl during this process.
X-Ray:
If you really need to know whether or not your pearls are real or fake or whether they are natural or cultured, you are going to have to pay someone, preferably a certified gemologist, to x-ray them for you. An x-ray will show the inside of the pearl, including variations in its density, the presence or absence of a parasite that might have caused the formation of a natural pearl, and the characteristic shapes of drill holes, if present.
The Tooth Test:
First, the "Tooth Test" while hailed by many as a surefire way to know if pearls are real or fake is not very reliable.
To use the tooth test, you rub the surface of the pearl over your teeth (make sure the pearls are clean). In general, a real pearl feels gritty while a (fake) faux pearl feels smooth.
The premise behind the test is valid...
Real pearls are made up of many layers of nacre that are deposited like sand on a beach -the slight irregularities in the nacre can produce a gritty or bumpy feeling against the teeth.
On the other hand, many synthetic nacres are applied smoothly onto perfectly smooth beads and these pearls would feel smooth against the teeth. The test is pretty good for distinguishing rare natural pearls from glass pearls, but it isn't as good at identifying cultured pearls, which have fewer layers of nacre and may feel more smooth.
If the pearls are dyed, the dye can fill in natural depressions in the pearls, making them feel smooth. Also, some synthetic pearls are made to look and feel like real pearls.
Mabe pearls may have a protective coating applied to them, which makes them smooth even though they may be genuine. The formulation of the nacre can closely resemble that of real pearls. The nacre could be applied to a natural base, like a shell bead, making this test completely unreliable.
The tooth test is a nice test, but don't base your judgement of authenticity on this test alone.
The Sun Test:
This is one of the easier and less costly tests to perform. You could perform this test outside in natural sun or under a very bright light. Unless they are very expensive (genuine pearls) they won't be perfectly matched under the sun.
In natural sunlight, you will be able to see variations in iridescence (orient) and color. If the pearls are perfectly matched for color and overtones, they are most probably fake.
If you are buying pearls from a seller who offers pearls that are perfectly matched, the cost of such pearls will be extremely high (into the thousands). These are pearls that are worth spending the extra money to have appraised by an independent gemologist.
Visual Test:
Fake (faux) look 'flat' in comparison to the real thing. There are exceptions, of course, with beautiful simulated pearls made by Swarovski and other manufacturers. Real pearls tend not to be perfect and may display bands in their nacre, bumps, ridges, or pits. They will vary in size and shape from one to another.
Genuine Pearls may have concentric ridged circles around them, which many buyers may take for marks from molding of a fake (which is seen in the exact middle of all the pearls on strands of some faux pearls).
Real Pearls come in many shapes, but they tend not to be perfectly round, so a "perfect sphere" should be suspect. Expensive genuine pearls may be round, but you will have other clues to help you make a determination. Some fakes are made to look irregular, and glass pearls often have flattened ends or slightly oval shapes.
In addition to visual clues, genuine pearls tend to warm to the skin much faster than glass pearls.
Drill Holes:
Real pearls tend to be drilled from both sides, to meet in the center. If you could see the cross section of the pearl, the hole may appear wider at the outside edge of the pearl than at the center.
Holes of real pearls tend to be as small as possible (with some exceptions), since the weight of a pearl affects its price (a larger hole translates into less weight and lower value).
Inexpensive real pearls may be lower in cost because the drill holes are not completely straight. Fake pearls often have larger, possibly straighter holes than real pearls. Some fakes are made to have smaller holes, so that they can be knotted like their genuine counterparts.
The Setting:
In a finished necklace or bracelet, real pearls are more likely to have knots between each pearl than faux pearls. Real pearls are more likely to have settings of gold, silver, or platinum than faux pearls.
You can examine clasps for stamps or engravings in the metal. Clasps tend to have safety mechanisms, like fish hooks, but some real pearls are sold with other types of clasps such as those which secure "hand in glove" (slide clasp) and the clasp is usualy made of 10KT., 14KT., 18KT. or Platinum.
Brand Names:
In the "World of Pearls" brand name does matter. Buyer's trust names such as Akoya and Mikimoto as names they can trust. However, when a brand name is not attached to the pearl or pearls you're contemplating purchasing, following the above "tests" (while not fool proof) can assist you in knowing the real thing from "faux".
Other guides relating to jewelry and gemstone buying which you may find helpful are as follows:
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