Not sure if that info fits that thread, but while you are at Pinna pearls ! I have just read in the march 2015 issue of our Revue Fran?aise de Gemmologie that the first cultured pearl of Atrina Vexillum has just been identified and studied.
The pearl is 10.6 x 9.6 mm (slightly button shaped), weight is 6.92 carats. The pearl has undergone in our national french lab X rays and tomography, revealing a bead inside.
I don't quite see the interest of such a culture in Atrina, but !...