Pearl luster has no objective measurement standard
I have been working with pearls for years, and there is one thing most buyers never hear: there is no universal grading authority for pearls. Unlike diamonds with GIA, pearl grades are entirely internal to each seller.
That AAA strand you bought? One seller's AAA is another seller's AA. Some add a fourth or fifth A to make their scale look more exclusive. It is all legal, all unregulated, and all decided by a single person looking at the pearl under a specific light.
Luster, the single most important quality factor, is still assessed visually. No spectrometer, no objective measurement. Nacre thickness on Akoya pearls? Also graded by eye. The only exception is Hanadama pearls, which are actually x-rayed at the Pearl Science Laboratory in Tokyo.
Science has already developed tools like OCT and UV-Visible spectroscopy that can measure luster objectively. The industry simply does not use them at the retail level.
Happy to answer questions. What do you actually want to know about how pearls are graded that sellers never tell you?