Uneven shape <=> uneven nacre distribution, methinks
.
I've only seen bad examples of these tailed pearls up-close so far, and the worst even have 'bald' spots with paper thin nacre over the nucleus, even if there is a thick-ish tail at the other end.
Also found rounds and drops claimed to be nucleated, and have no idea of the depth of nacre on those.
Just a thought...
In fact, this thread is the first time I hear the question! The few reports about nucleated freshwater pearls found over the web are apparently too taken by the surprise of the mere existence of such pearls, to care about nacre thickness.
