Pearl lustre is the measurement of the quality and quantity of light that reflects from the surface and just under the surface of a pearl.
Pearl Lustre is the Shine That Gives Pearls Their Beauty
Lustre 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. Pearls with low lustre appear white or chalky, rather than brilliant and shiny.
High Pearl Lustre Comes from More Nacre
High lustre 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 mussel or oyster. Pearls with a high lustre have more nacre and are generally valued much more highly than pearls with a lower lustre or less nacre.