Nora,
 
You are correct, both shells have been polished.  Many cowries will show a
purple dorsum when the outer layer is polished away.  The most commonly seen
are purple Cypraea caputserpentis and C. moneta -- I know them well as I
have examples of both that I purchased back in my younger days.  But as for
throwing them out, that is not really necessary as they can make an
interesting display next to a shell in the original condition.  I have the
African turban (Turbo sarmaticus), button top (Tectus niloticus), and west
Indian top (Cittarium pica) all with both polished and unpolished specimens.
It makes an interesting display.  People who know little about shells like
the polished specimens, of course.
 
But it is in fact all a matter of degree.  This last year I saw specimens of
Conus striatus that had been polished for sale at a store in California.  I
had never seen a polished cone and at the time could only think about what a
waste as some looked like pretty nice shells.  Now I kind of wished I had
picked one up as another example of a polished shell.  An interesting
display could start with a C. striatus with the periostracum, then one after
cleaning in bleach, then one with a "dressed up" lip, then a polished
specimen; from scientific specimen to collector specimen to square filler
specimen to tourist specimen.  Just a thought.
 
Tom Eichhorst in New Mexico, USA