Thank you kindly ladies!
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As for the shells, we have seen no evidence that any other shell than the Hyriosis cumingii, or the Hyriosis cumingii/Hyriopsis schlegelii hybrid are being used in fresh water cultivation. It is probable that these pearls come from such a hybrid.
Some people selling similar pearls have taken to calling them "Ikecho", (which is the name for Hyriopsis schlegelii in Japanese). However only hybrids have been used in Japan since about 1990.
Sincerely, SARAH