Real Pearls vs. Fake Pearls 
Both real pearls and fake pearls can be beautiful, but sometimes it's important to know which type you have. If you buy pearls, you could identify fake pearls from their descriptions: fake, simulated, faux, glass, plastic, synthetic, resin, artificial, manmade. There are various types of genuine pearls, which could be broadly classified into natural pearls and cultured pearls. Real pearls may come from fresh water (rivers, lakes, ponds) or saltwater (sea or salt marsh) and from a variety of mollusks.

What complicate the issue is that pearls could be partly natural and partly manmade. There are glass beads, plastic beads and mother-of-pearl shell beads which have had treatments applied to them, such as coatings or faceting.

Here's a list of tests your can use to distinguish between genuine pearls and all the various fakes.

Tests for Real Pearls and Fake Pearls 

 
• The Sun Test
This is my favorite test. Sometimes you can tell just by looking at the lustre (how the pearl shines in the light) most faux pearls tend to be in pure white and look too perfect while natural pearls may have a tinge of yellow or pink under natural light. This can sometimes apply to their shape but it depends on the quality of the pearls you buy. It involves taking your pearls be exposed under the sun or very bright indoor lighting. Unless they are very expensive, genuine pearls won't be perfectly matched under the sun. You will be able to see variations in their iridescence (orient) and color. If the pearls are perfectly matched for color and overtones, they are most probably faux. It costs about $150 to have pearls tested, as opposed to several thousands of dollars for the type of pearls that warrant the test.

 
• Destructive Tests
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 reveals the nature of its drill hole, if present. Also, you'll destroy the pearl. It isn't recommended.

 
• X-rays
If you really need to know whether or not your pearls are real or fake or whether they are natural or cultured, you need to pay a professional, preferably a certified gemologist, to conduct a X-Ray test on them. An X-Ray test can give you a picture of 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
Let me preface my remarks by stating an important point: This test is NOT reliable! To use the tooth test, you rub the surface of the pearl over your teeth. Supposedly a real pearl feels gritty while a faux pearl feels smooth. The premise behind the test is valid: Real pearls are made up of layers of nacre that are deposited rather like sand on a beach. The slight waves and irregularities in the nacre can produce a gritty or bumpy feeling against the teeth. On the other hand, many synthetic types of nacre are applied smoothly onto perfectly smooth beads. 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 smoother. 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 judgment of authenticity on this test alone. If you think it's reliable, I swear I could find pearls that would fool you, both real and faux.

 
• Magnification
As is true with diamonds, magnification reveals a lot about the quality of a pearl. You can see the characteristic ridges and irregularities of real pearls or the grainy smoothness of fakes. You can examine drill holes to see the interface between the nacre and what lies beneath it. You can read any writing on the clasp or setting.

 
• Other Visual Cues
Fakes tend to 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 have bands in their nacre, bumps, ridges, or pits. They vary in size and shape from one to another. Genuine pearls may have concentric ridged circles around them, which inexperienced people 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 suspected. Expensive genuine pearls may be round, but you need other clues to help you make a judgement. 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 the skin much faster than glass pearls would. Resin or plastic pearls tend to feel somewhat warm upon first contact.

 
• Density
Density is the mass of an object as a function of its volume. For a given size, real pearls are heavier than plastic, resin, or hollow glass pearls. Good glass fakes will have the same density as that of real pearls. Light pearls are fake but you cannot distinguish real from faux simply on the basis of density alone if the pearls are heavy.

 
• Hole Characteristics
Check the drill holes. Real pearls are drilled as small as possible to maintain their value, fakes often have larger holes. If you see the cross section of the pearl, the hole may appear wider at the outside edge of the pearl than at the center (which can make stringing poorly-drilled pearls very challenging and is one reason many people won't restring pearls that they didn't sell). Some fakes are made to have smaller holes, so that they can be knotted like their genuine counterparts. Inexpensive fakes may have holes of widely variable sizes on a single strand. Also, the pearl surface, or nacre, will often flake off around the holes of fakes.

The nacre of fake pearls is more likely to flake away near the drill hole than on a cultured pearl (it won't flake on a natural pearl). Either the flaking or the sight of a clear inner bead may clue you in to a fake. Most fakes have pearl-colored centers, so the center color may not help you. The holes of fake pearls often form a shallow bowl shape, while the holes of real pearls are more likely to be flat. Examining the hole is also a good way to detect signing of dyeing.

 
• Quality of Jewelry Setting
If you see the Mona Lisa in someone's home, you can be pretty sure it isn't the original piece of artwork. Similarly, you can gain valuable clues about a pearl's authenticity by looking at its surroundings. 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 or for magnetism (indicating the clasp or setting contains iron as opposed to a precious metal). Clasps tend to have safety mechanisms, like fish hooks, but some real pearls are sold with other types of clasps, usually as an accommodation for a person who would have difficulty opening/closing the more secure clasp. However, insecure clasps are not usually seen on good pearls. It isn't a hard-and-fast rule. Some fakes have high quality settings. Some genuine pearls have cheap settings. Even so, the setting can provide clues to help you reach a conclusion.