Hello again Burt,

I just noticed your note about the other turban specimen.  That shell is
undoubtedly the same species (Turbo marmoratus), but in its natural
state.  The opalescent one has had the outer green and brown layer
buffed off, to reveal the underlying nacreous (pearly) layer, followed
by polishing.  Such "pearled" specimens are frequently produced as
decorative items.

Regards,
Paul M.