Tahitian pearls are named after the island of Tahiti and they are highly prized for their beauty and the unique charm of their darker tones, thanks to the island's black-lipped oysters that produce black, gray and green-black pearls. Recognizing their value, pearl farmers used the pearl culturing techniques pioneered by the Japanese to cultivate Tahitian pearls. Since then, Tahitian pearls have become one of the most sought-after gems in the world.

Factors to consider when choosing quality Tahitian pearl necklaces, in order of importance:

Nacre thickness ? ideally, the minimum thickness of nacre in Tahitian pearls is 0.8 mm. A thicker nacre covering means that the pearl's luster will last long. Thinner nacres have a tendency to wear off easily. Tahitian pearls which are thinner than the required minimum are not exported from French Polynesia.

Surface texture and quality ? Tahitian pearls, like all pearls, are graded according to the quality of its surface. Pearls could have thick nacres but if the surface is flawed, their value decreases. Flaws include bumps, spots, discolorations, scratches and other irregularities. The more a pearl has, the less valuable or lower-grade it becomes.

To rate the quality of Tahitian pearls, jewelers use a grading system, consisting of A, B, C and D as the marks, with A-grade pearls being the highest quality. A-grade pearls have the cleanest, nearly flawless surface, virtually free of spots, discoloration or bumps. These pearls have a high luster and reflective quality.

B-grade Tahitian pearls have slightly (but still imperceptible) imperfect surfaces, with only 30% of the surface affected by flaws. C-grade Tahitian pearls have medium luster and imperfections are already quite perceptible while D-grade pearls are those with 60% of its surface affected by quite obvious imperfections, including scratches.

Luster ? one quality of pearls that sets it apart from other gemstones is its unique brilliance. As opposed to other gems like the diamond, for example, pearls shine with a softer light. And since they reflect light, they also look like mirrors, reflecting everything in their immediate surrounding.

Generally, the thicker the nacre, the more brilliant and mirror-like the shine. Good quality pearls are clear and almost metal-like in their sheen.

Size ? the largest Tahitian pearl ever produced is 25mm, although this in not a round pearl. The largest round Tahitian pearl is at 21mm. Generally, the bigger the size of the pearl, the higher its value is. Most Tahitian pearls are in the range of 8 to 14mm, although a few are measured at 16mm.

Shape ? pearls follow the shape of the irritant or nucleus that enters the mouth of the oyster or mollusk that produces it. But to meet jewelry standards, most pearls are produced following three major categories: round or spherical, symmetrical and baroque.

Spherical pearls are those that are round or nearly round. A perfectly round pearl, whether natural or cultured, is quite rare and therefore highly prized. Spherical pearls are the most familiar and popular shapes. Nearly-round pearls are categorized as sphericals, although they may be slightly elongated or flattened.

Symmetrical pearls are so-called because if you cut one in half, each piece will mirror the other. Included in this category are oval pearls, which taper at the ends and are wider at the center; button pearls, which are slightly more flattened and often used for earrings; and drop pearls, which look like teardrops or pears, usually included in jewelry as pendants.

Baroque pearls may also be semi-baroque and include all other shapes that cannot be classified as either spherical or symmetrical. These are the favorite pearls of artists because of their irregular shapes. Baroque pearls may resemble sticks, studs and even crosses.

Other notable variations in Tahitian pearls are those that have ?rings? ? tiny grooves that encircle the pearl's surface. For this reason, these pearls are called circled or ringed, although this quality is only used as a descriptive device, to distinguish one pearl from another. For example, a ringed pearl may be described as a ?ringed oval? or ?circled drop?.

Color ? when Tahitian pearls are mentioned, the first thing that comes to most people's minds is black pearls although they do come in other colors, albeit in darker tones. Some of their interesting colors are gray, green, brown and blue.

The pearl's color is determined by two things: the main body color and its overtone. A pearl can have one or more overtones. The main body color is the predominant color of the pearl while the overtone is the hint of iridescent color that is seen on top of the body color and can affect or enhance the dominant color of the pearl. Pink, gold, blue, silver and purple are some of the most common overtones.

A combination of the body color and overtone is used to describe a pearl. For example, a greenish black pearl is called a peacock while a yellowish gray is called a champagne.

When buying Tahitian pearl necklaces, keep the above considerations in mind. Aside from quality, you might also want to consider design and setting. Don't be afraid to ask questions and information in detail. If the jewelry shop is reputable, they will be more than glad to answer your questions.

On your own, shopping for real Tahitian pearl necklaces can be quite a challenge. To help you make your decision, here are a few tips on real Tahitian pearls:

- they have overtones over the main body color. Fake or imitation pearls don't and their color looks flat. Real Tahitian pearls also glow with a faint reddish color when examined using crossed filters.

- Tahitian pearls feel cool, although they do adapt to your body temperature. Imitation pearls maintain the same temperature as their surroundings, so when they are touched for the first time, they won't feel cold.

- To test for real Tahitian pearls, hold them in the palm of your hand and take note of their weight. They are heavier than imitation pearls. If you rub Tahitian pearls across your teeth, they will feel grainy or coarse and not smooth. Their holes are also smoother around the edges.