Most pearls available today are cultured, meaning that the mollusk is purposely inserted with an irritant or “nucleus” from which it creates a beautiful pearl.

An excellent way to understand the difference between natural pearls and cultured pearls is simply to think of the natural pearl as a result of the mollusk working alone, while cultured pearls are a product of nature being helped by science.

Cultured pearls aren't “fake” like many people may believe. The culturing process takes from two to three years and the process is a very delicate one. Pearl farmers have little control over what the finished pearl will look like or if the mollusk will reject the nucleus.

Not all pearls are of fine quality, or even desirable at all. The end result of the pearl culturing process is ultimately up to nature. 

A pearl's quality is determined by many factors, including luster, color, size,  shape,  and surface perfection.

Luster
Luster is very important and one of the first factors to consider when choosing pearls. Luster is an intense brightness quality that illuminates from within the pearl itself rather than just being “shiny” like in imitation pearls.

This intense brightness results from light being reflected throughout the numerous layers of the pearl's nacre. High quality pearls have a sharp contrast between the brightest area where direct light is hitting the pearls and the shaded area, which creates an illusion of a “ball within the pearl.”

Color
The color of the pearl is also very important. When evaluating pearls, note their body color and overtones. The most desirable and rare white pearls have “rose” colored overtones.

Cream colored pearls are less expensive because they are less rare. Also, pearls can have tones rated in intensity. Pearls with darker tones are more desirable and expensive compared to lighter toned pearls.

Pearls are available in many natural body colors, including white, black, blue, pink, gray, gold, and green. Distinctive colored pearls are rarer and more difficult to find. Ask a qualified gemologist to check the pearls to make sure they haven't been dyed, especially for more expensive pearls such as the black or golden varieties.

Size
Pearl size should also be taken into consideration. Larger cultured pearls are rarer and more expensive. Akoya pearls over 7 ½ millimeters are much more expensive, and prices rise dramatically with each 1/2 millimeter over 8 millimeters. South Sea and Tahitian pearls also have a large increase in price when the size is over 15 millimeters.

Shape
Consider shape when choosing pearls. There are three pearl shapes: spherical, symmetrical, and baroque. Spherical pearls are the rarest and most desirable. Symmetrical pearls include teardrop or pear-shaped pearls. These are desirable, but usually less expensive than spherical pearls. Baroque pearls are irregularly shaped and usually the least expensive. 

Surface Perfection

Examine the pearls’ surface for blemishes. Although it's best to check for most pearl characteristics on a light background, you should check for blemishes on a dark background. Be sure to check in both intense and diffused light.

No pearls are perfect, and small blemishes are acceptable as a trade-off for a more important quality like nacre and luster. Pearls with higher luster conceal any blemishes more effectively. 

Matching
One more factor to consider when evaluating pearls is the precision in matching the pearl quality in a string of pearls. It is very important to take all of the above factors in consideration when matching the pearls.

Graduated pearls also require careful matching. Pearl matching affects the value of the item because when pearls are not matched properly it detracts from the appearance of the jewelry. Also, check that the pearls are all drilled through the center so they lay properly. Pearls that were drilled off-center will not lay correctly which reduces the value of the piece.