Fortunately, most organic solvents, like those which have been mentioned as
diluents and vehicles for mineral oil, are harmless to shells.  However,
such solvents are highly destructive to most of the materials that might be
used to coat a shell, including oils, waxes, plastics, varnishes, shellacs,
lacquers, acrylics, etc.  Five minutes in a good general solvent like
mineral spirits or ordinary paint thinner will usually tell you if the shell
was coated.  When  you remove the shell from the solvent, wipe it with a
towel or similar cloth.  If it feels sticky or slimy as you dry it, that's
one indication of a coating in the process of dissolving.  Once the shell is
thoroughly air-dried, see if it looks any different than it did before
soaking.  If it is less glossy, or different in color, then it was probably
coated with something.  However this method is not foolproof, since not all
solvents will dissolve all coatings.  Some coatings are more difficult to
remove than others.  For example, if an epoxy compound was used, it can be
very difficult to dissolve - actually, impossible with any solvent you are
likely to have at home.
        As for Cypraea teulerei, the color and markings are quite variable.
I have some specimens like you described Nancy, whitish with reduced
markings.  They are not dead collected, but they do look somewhat
"abnormal".  I suspect they either formed abnormally from environmental
causes, or they are old "geriatric" individuals.  But I'm not certain.
        Regards,
        Paul M.