I came across a report on two nacreous baroque pearls (10 and 18 mm) that apparently came from the shallow water Mytilid Modiolus capax from Baja. They have a greyish purple color similar to the inside of the shell and would fit easily in the live animal. The article is: Natural Saltwater Mussel Pearls, C.Y.Wentzel, 2004, Gems and Gemology, The Quarterly Journal of the Gemological institute of America, V0l.40 No.4.
If anyone wants, I can send a copy at your request.
Allen Aigen