Nacre Quality

As you've read, nacre quality is determined by its thickness and the regularity of its layers. The GIA Pearl Description System classifies three categories of nacre quality:

Acceptable—Nucleus not noticeable; no chalky appearance

Nucleus visible—Shows evidence of bead nucleus through the nacre and scattered light reflections called blinking when rotated

Chalky appearance—An obviously dull appearance

The nacre thickness of spherical bead-nucleated cultured pearls is stated per radius, which is the distance from the pearl’s center to its outer surface. This gives a more accurate picture of nacre thickness than a simple diameter measurement would provide. For non-spherical bead-nucleated cultured pearls, the trade measures nacre thickness on one edge. Off-center beads can lead to inconsistent nacre thickness measurements.

For its Pearl Reports, the GIA laboratory X-rays bead-nucleated cultured pearls and states the per-radius nacre thickness in millimeters. X-rays are the only reliable way to determine nacre thickness without sawing a pearl in half. Candling—examining a pearl in front of a concentrated light source—can detect the presence of a bead nucleus and even give an indication of nacre thickness, but it won’t yield a precise millimeter measurement.

Nacre that’s too thin to conceal the nucleus diminishes a cultured pearl’s value, contributes to poor luster, and threatens durability. 

Robert Weldon/GIA, courtesy Armand Asher Pearls
Nacre quality is determined by the layered structure of the aragonite crystals and the nacre thickness. If a cultured pearl has attractive luster with good reflection depth, it most likely has good nacre quality.