The pearl, symbol of purity, virtue and modesty, is also one of the most precious types of jewelry. Technically known as "organic gems" since they are formed by shellfish, pearls have been harvested and worn for more than 4,000 years. The way they are acquired - and their appearance - has changed dramatically over time, especially in the past hundred years, but pearl jewelry nonetheless continues to be a classic.

A pearl is formed when an irritant, such as sand or a parasite, becomes lodged in the shell of an oyster. The oyster deposits layers of a semi-translucent crystalline material called "nacre" around the intruder, where it builds up in layers like the rings of a tree. This process of building up can continue for years, resulting in a pearl. In nature, pearls take many years to develop and often have irregular shapes, ranging from slightly off-spherical to twisting, bulging shapes called "baroque." In any shape, natural pearls are rare and very costly.

Around the beginning of the 20th century, it was learned that if a sphere of material was placed into an oyster and the oyster stimulated correctly, the oyster would coat the sphere with nacre, creating an almost perfectly round pearl. The longer the pearl remains in the oyster, the larger and more valuable the pearl becomes. These are called "cultured" pearls. Almost all pearls used in jewelry today, including all the pearls sold by Zales, are cultured pearls. Pearls are cultured around the world today, and different types of oysters - or mollusks in freshwater - raised in different environments create cultured pearls with different sizes, colors and other qualities.

Cultured pearls are rated on five different qualities:

Luster and orient: Luster is the sharpness and intensity of reflections on the pearl's surface, and orient is the iridescent colors one sees within the pearl. The higher the luster and orient, the more valuable the pearl.

Color: Color describes both the main color (usually white, black or yellow) and the undertone (often pink, rose, or even green.) In addition, pearls can be dyed any shade to meet personal preference.

Cleanliness: Describes how many imperfections are found on the surface of the pearl. Some imperfections are expected on all real pearls, natural or cultured, but the fewer and less noticeable they are, the better.

Shape: In general, spherical pearls are the most prized. However, it is common to find small imperfections of shape, which can sometimes be desirable as long as they are symmetrical. For example, teardrop shaped pearls are often used in earrings and pendants.

Size: Cultured pearls are sold by diameter, measured in millimeters. In general, larger cultured pearls are rarer and more costly. Price rises significantly with the size of a pearl.

In addition, one should look at how well matched pearls are when combined in jewelry, such as on a necklace or in earrings. You should look for pearls that are essentially the same size, color, shape and luster. Keep in mind that the more well-matched pearls there are in a piece of jewelry, the higher the cost. That's why pearl earrings cost more than two individual pearls.

When you buy cultured pearls, keep in mind that they are fairly soft as gems go. They can be scratched easily if they are bumped into things, and can also be scratched by other jewelry they might come into contact with. Keep them separate from harder gems in your jewelry box.