Paul,
 
Nice explanation of how pearls are formed, though you left an inkling of
doubt as to what species have pearl-forming ability.  Fact is, all
shelled mollusks have the ability to produce pearls...gastropods,
bivalves, chitons, even argonauts. But the two most unusual pearls I've
seen lately are from Haliotis rufescens and Nodipecten subnodosus. The
haliotis pearl is about 7mm, a near-perfect sphere, and a lustrous
blue-green. It now resides on a lecklace in my wife's jewelry box. The
pecten pearl is an 8mm purple orb, formed in the wide purple margin of
the N. subnodosus. A few years ago, I also saw a small pink pearl
(probably 2mm) taken from a specimen of Stenoplax conspicua, on of
California's larger chitons. About the same time, I saw a small,
brown flatish pearl which was produced by Argonauta hians.
 
Kim Hutsell
San Diego