The Nacre of Pen Shells (Pinnidae) seems to be more "pure" (less protein content) than that of the Pearl Oysters (Pteriidae). It would be interesting if a Gem Lab did an analysis on water and protein content and compared it to other species. When I refer to this I am talking about the nacre ON-THE-SHELL (not of pearls)
Anyway, it seems like this purer nacre has issues with dehydration...a tendency to loose water, thus the pearls just crack. I have yet to see a Pinnidae pearl without a crack. When I see one without a crack I just think: it will crack in the future.
Now, on natural loose pearls what we've seen is that the pearls have large quantities of protein, and the pen shell's periostracum protein is more fragile than that of the Pearl Oyster's. So, I believe we have a "double whammy" here.
I could be wrong of course, I don't think I've seen more than a hundred natural pen shell pearls (I own one from Pinna rugosa). And all our Pen Shell Mabe cracked... no matter what we tried on them. Just excessively fragile.
Here I have some photos of Natural Pen Shell Pearls (Atrina maura): all with cracks or other severe defects. Also, the "best" Pen Shell Mabe we had (the only one we kept) the we produced in 1996.