Jeremy,

Based on what I learned in Tahiti recently, I'll throw in my two cents worth here.

I don't know if the clam shell is a darker colour than the Mississippi shell - if it is, and it's cheaper, it could explain the greater frequency in black pearls as, unlike for lighter coloured pearls, the nucleus for a black pearl does not have to be as light in colour so the farmers may think they are economising, without realising that they are causing a problem further along the chain.

I am attaching a photo of shell nucleii that were used on the farm I visited. Any comments about whether they are clam shell or Mississippi would be appreciated by more than just me, I'm sure.

I'll also say that I have had pearls drilled that I bought from that farm and they've been fine.

Note, the pic shows a plastic bag of nucleii in the background and a few loose ones in my hand.

Attached Images