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 a greater luster than freshwater pearls. Pearls with low luster appear white or chalky, rather than brilliant and shiny.
When light falls on pearls, it is reflected off the surface. This results in a shine which is called larger the pearl, more the luster. within the pearls' nacre. The thicker the layer of nacre, the deeper the luster or shine.