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A string, a ring, a choker-pearls can be a bride's best friend. Here, find out everything you need to know about this classic bridal accent.

What is a pearl? Scientifically speaking, a pearl is formed naturally when an irritant—a grain of sand, for example—gets trapped inside the thin tissue of a mollusk's inner shell. To protect itself, the mollusk releases a substance called nacre, which attaches to the intruder and forms a smooth, hard layer around it. As more nacre is released over time, the layers accumulate, creating a pearl.

So-called "cultured" pearls are formed in the same way, except the irritant is purposefully placed inside the mollusk. Ninety-nine percent of pearls sold today are cultured.

No two pearls are alike. But to glance at an exceptional strand—in which all of the pearls seem identical in shape, size and color—you'd never guess it. Nacre can range in color from white to black, while size varies from the tiniest bead to some South Sea pearls that boast widths the size of a quarter.

Pearls of Wisdom

You can save a lot of shopping time and confusion by knowing what to look for. Your first key in determining a high-quality pearl is its luster—the way a pearl seems to glow from within as well as the way light plays over its surface. A high-luster pearl will have strong, sharp reflections of light, and a remarkable contrast between the bright and dark areas on its surface. These pearls are not always easy to find, but are worth searching for. A good test: Can you see your reflection in the pearl? How clearly? A low-luster pearl will have a milky, chalky surface (almost like a bead) with very little reflection. A highly lustrous pearl, by contrast, plays with the light with every move.

Next, examine the pearl's surface. Note any disfiguring spots, bumps, dimples or cracks on the "skin," which should be clean and smooth, with few or no obvious imperfections. Granted, a blemish-free pearl is rare. But if your eyes are drawn to flaws now, so will everyone else's once you're wearing it.

Take a look at shape. Pearls run the gamut, from oval and pear-shaped to round, as well as more obscure shapes. Depending on what type of jewelry you're buying, shape can be more or less important. Perfectly round pearls are hard to come by (and their price tag will confirm that!). But for a classic necklace or striking ring, the more uniformly round pearls you can find, the better.

All of these characteristics—plus the size—determine a pearl's cost. The size of a pearl is measured in millimeters, and just one millimeter can increase the price by a whopping 30% to 50%. Surprisingly, a pearl's creamy hue does not usually affect its value, simply because most pearls—with the exception of Tahitian and South Sea—receive color-enhancing treatments before they're sold.

Get Stranded

Pearls are a classic wedding accessory. If you're investing in a necklace, make sure the pearls are well-matched. The best strands will be strung on silk, with individual knots between each pearl. The knots keep the pearls from rubbing together and prevent you from losing all your pearls if the strand breaks.

The most common strands of pearls are a choker (14 to 16 inches) and a princess (17 to 19 inches); a collar refers to three or more stacked strands that lie snugly on the middle of the neck. All are classic and versatile. As for the pearls themselves, there are some drastically different types on the market. Here are some of the most popular:

• Akoya: These beauties from Japan account for 50% of the industry's cultured pearl business because of their variety of colors and shapes. These are the most commonly used pearls for classic strands.
• South Sea: Known for their extraordinary size and luster, these pearls range in hues from white to gold. They're the rarest—and the most expensive.
• Tahitian: Commonly called "black" pearls, their striking colors actually vary from light gray to deep black, with shades in between ranging from pistachio to purple to greenish blue.
• Freshwater: Cultivated from freshwater mussels and produced in abundance, these are less expensive than sea-oyster pearls.
• Mabe: Grown against the inside shell of an oyster, instead of in the body, they're often flattened on one side or have an irregular, pebble-like shape.