QUALITY FACTOR ONE: LUSTER

Lustre is considered the most important quality factor in pearls. Lustre refers both to a pearl's brilliance -- the way its surface reflects light -- and its inner glow: the way it refracts light. A pearl's lustre is generally evaluated in terms of "high" to "low," with grades of "medium" in between.

High-lustre pearls are bright, and have a deep-seated glow. They reflect objects near them clearly. Though high-lustre pearls usually have a thick nacre coating, thick nacre doesn't always guarantee a pearl will have high lustre. This is true because genetic imperfections in some oysters don't allow them to secrete nacre in perfect patterns that result in high lustre.

Low-lustre pearls, on the other hand, have low reflective and refractive qualities. They may appear too white, or dull or chalky, and they usually have only marginal nacre thickness.