How to value a pearl?
Pearls have 5 value factors:

Size:
The size of pearls is measured in millimetres. The pearls that have a bigger size are more precious due to their rarity. The size of a pearl depends on several variables: the size and health of the mollusc, the size of the nucleus and the amount of time spent building layers of nacre. The climate and nutrient conditions of the water in which the mollusc is grown also have an influence on the pearl size.

Shape:
The shape of pearls is different. They can be spherical, symmetrical or baroque. Very few pearls are round. They might be near-round, button-round, semi-baroque, baroque, coin, circle and drop pearl.

Colour:
Cultured pearls present a broad palette of subtle hues. Pearl colour has several characteristics: the body colour or dominant, overall colour of a pearl, the overtone (one or more translucent colours) and the orient (rainbow colours shimmering on the pearl’s surface).  All pearls display body colour but not every pearl possesses overtone or orient.

Luster:
The luster is one of the most appreciated characteristics of a pearl. It represents the intensity of the reflected light, the “inner glow” of the pearl. A pearl with excellent luster will act as a mirror.

Surface quality:
The surface quality of a pearl refers to the degree of irregularities on the surface of the pearl. They are also called imperfections or blemishes like spots, abrasions and bumps. The value of a pearl is higher as blemishes are less visible. Even so it’s very rare that a pearl has no imperfections.