Island Pearls is a most interesting site. Paul Cross is an American who is very cutting edge in pearl technology. Also very politically correct.
Also read the following page for a 1998 story re stealing mussels from the MI River Basin and selling them to the Japanese.
http://www.islandpearls.net/article3.htm
Also above article mentions the rarity of these shells and that was 8 years ago. The above article really ties in with a recent theft reported from the Muskogan river
Mussels stolen from Muskingum River