One less scientific way to judge thickness of the nacre on a
cultured pearl (and its overall quality) is to spin it in direct
light and look for uneven reflections from the mother of pearl bead
it was formed around. A pearl with thick nacre will show none or
almost no “blinking” as it is turned, while one with thin nacre will
blink like a lighthouse.
If it is a strand of cultured pearls, hold it with one end in each
hand and twist the entire strand so as to rotate all of the pearls.
In the case of a single loose pearl, roll it back and forth in a
trough of some kind, folded paper works well.
The opacity of the nacre in addition to its luster, orient and color
is more important to a cultured pearl’s overall quality and value
than the thickness, although the two generally go hand in hand.