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Pearl Luster
Luster is essentially the reflective quality or brilliance of the surface of the pearl nacre. The more lustrous the pearl, the more it shines and reflects light and images. In general, saltwater pearls tend to have greater luster than freshwater pearls. Pearls with low luster appear white or chalky, rather than brilliant and shiny.

High luster results at least in part from the amount of nacre deposited to form the pearl, which in turn is a function of how long the pearl has had to develop within the oyster. Pearls with a high luster are generally valued much more highly than pearls with a lower luster. A bit of an exception to this value rule are the keshi or "poppyseed" pearls. Because they are 100% nacre, keshi tend to be the most lustrous of all pearls, yet they are often less expensive than cultured pearls.

From Pearl-guide.