[CortezPearls]

There are many sources of shell to use as nuclei (Pinctada maxima, the endangered Tridacna gigas and many Unionids) as well as the "famous" Bironite material.

The shell of several species of Missisippi Freshwater Pearly Mussels (Unionidae) are considered a de-facto standard since it was the Japanese that introduced this standard (and controlled the Pearl Industry for most of the 20th Century).

But you can use other sources...I don't doubt many Chinese are producing nuclei from their shells and we know -thanks to Jeremy Sheperd & Doug Fiske- that they are also employing the endangered Giant Clam (Tridacna gigas) to produce nuclei for the recent production of "Fireball Pearls".

In most instances, the process for production would be the same: cut strips of shell, then square them and finally grind them into spheres. Hope this photo illustrates the process...