The characteristics taken into account in the classification of pearls are the following:

Surface. The more flawless the surface of the pearl is, the higher it will be valued. However, a flawless pearl is only a one in a million occurrence, as pearls are the result of a natural process and an oyster will usually leave some sort of unique mark on the finished pearl. The number, nature and location of surface characteristics (abrasions, bumps, chips, cracks, etc.) can affect the value of any pearl.

Luster. The first and most important definition for a pearl’s beauty. Luster, the intensity of light refelcted from or just below the surface of the pearl,  should be your primary focus when judging quality. To recognize finer luster, look at the clarity of images that are reflected in the pearl’s surface. The closer to a mirror image you see, the better the luster. Pearls with fine luster also appear to glow warmly from within.

Color. Unlike the other factors, color has little influence on the actual value of a pearl, except in the case of popularity. Essentially, the color of the pearl you desire is a matter of personal taste. Traditionally, there is a rather consistent demand for the classic silvery white pearls and the glowing gold shades. The rarest and most desired are the white “rose” colored pearls.

 
Size. Typically, the larger the pearl, the more rare and costly it tends to be. But fine quality pearls can be small, and low quality pearls can be large, so a pearl's ultimate worth depends on how it combines the complex mix of value factors. The size of a pearl is measured across its diameter, and range from 1.8mm to 6mm in Natural pearls and from 3mm up to the very rare 18mm in Cultured pearls.

Shape. The rounder a pearl is, the rarer and more valuable it is. The goal is for a perfectly round pearl, though there is much more to pearls than an ideal orb.