In this photo from the Gemological Institute of America, from top to bottom, these Japanese saltwater cultured pearls show excellent, very good, good, fair, and poor luster. The differences in the sharpness of the reflections between each adjacent strand are subtle but still noticeable.
Luster is one of the most significant pearl qualities. The luster of a pearl is much more than the luster of an object that is just reflective of the light on its surface. Luster refers to the light reflected off the internal layers of a pearl’s nacre. It’s a glow from within the pearl. The pearl’s layers of nacre is a translucent substance that is layered like bricks. Natural light travels through these layers and reflected back out in a prismatic lustrous rainbow in the best examples. The quality of the nacre is what creates the quality of the pearl’s luster.