According to the Gemological Institute of America (GISA), the major factors that determine the quality, and hence the value, of pearls are as follows:

Pearl Luster
The luster is the shininess of the pearl. The higher the luster, the sharper the reflection in the pearl and the more prized the pearl.

Pearl Color
Pearls can come in several colors. Other than white, pink, silver, gold and black color, a high-quality pearl will have a second overtone. The overtone might be reddish, greenish or bluish color. White-pink and white-silver overtones are the most valued colors on the market today.

Pearl Nacre Thickness
The thicker the nacre, the smaller the nucleus, hence the higher the quality of the pearl. Nacre thickness can only be measured by an expert. Cultured saltwater pearls have a nucleus implant, thus the thickness of the nacre will be thinner than that of freshwater pearls.

Pearl Shape
Spherical-shaped pearls are the most expensive on the market due to their rarity. Most pearls are not perfectly round because of the uneven-layered nacre.

Pearl Surface
Since pearls are naturally formed, all pearls have some imperfections. The fewer spots or lumps that a pearl has, the higher the cost of the pearl.