1.) How/where it is produced
As previously mentioned, natural pearls are the most rare and most expensive, being that only a select few remain on the market. With that being said, most pearls on the market are the product of an organic and natural process with the help and aid of humans, which is referred to as pearl cultivation or harvesting. Also, saltwater pearls tend to be more valuable than freshwater pearls.
2.) Nacre
The quality the nacre is a major factor which contributes to the assessed value of the pearl. Nacre thickness refers to the amount of nacre that surrounds the nucleus of the pearl. Pearls which have a greater nacre thickness are considered more valuable than pearls that have a lower nacre thickness. One reason for this is because nacre thickness indicates how long it took to produce the pearl, since each layer is added organically by the mother mollusk. The older the pearl, the greater the nacre thickness, and the greater the value. Determining nacre thickness takes experience and skill. An informed consumer can tell the difference by evaluating the luster and weight of the pearls. Pearls that have brilliant luster tend to be the result of greater nacre thickness, since the nacre is what causes the luster. They also tend to be heavier since the nacre adds weight to the pearl.
3.) Surface
When buying pearls, it is also important to scrutinize the surface of the pearl for marks, bumps, or other imperfections. Since pearls are the result of a natural process, they will rarely ever be perfect and sometimes, even, the imperfections can add to the uniqueness of the pearl. However, if buying a strand of pearls, some imperfections lower the value of the pearls since it might draw unwanted attention away from the rest of the pearls.
4.) Shape
In general, the rounder the pearl, the higher the value. Some pearls, such as baroque pearls, are valued for their organic shape. However, pearls such as these are still generally not as expensive as pearls of same weight and luster that are round.
5.) Color
The most common colors of pearls exist between pale shades of white, champagne, rose, silver, and sometimes green. Tahitian pearls are the only pearls that nare naturally black. Natural colors can come in various shades and hues of these colors. When selecting your pearls, it is also important to know if the color of the pearls are natural or color treated. Color treating pearls is a common practice, and therefore, can create pearls of nearly any color. Since this is an artificial process, it is important to know if the pearls you are buying are natural or color treated, as color treated pearls are typically less expensive. In choosing your pearls, preferred tones are based on individual tastes and preferences. In general, rose colored overtones are most flattering for those with pale skin. Cream and golden hued pearls are most flattering for those with tanned and darker complexions.
6.) Size
If all the other factors are the same, the final factor that should be assessed is the size. Pearls are measured in millimeters and a single millimeter difference can substantially increase increase its value.