Linda, that's a tough one.  Rust stains are probably the most difficult type
of stain to remove from a shell. This often happens to shells that are taken
by commercial trawlers, and which have been allowed to lie on the deck for a
time.  The problem is, shells that have been in contact with iron, in the
presence of moisture, actually react with the iron chemically.  The iron
replaces some of the calcium in the shell, so rather than being a simple
stain lying on the shell surface, the resulting iron compounds actually
become part of the molecular structure of the shell.  In some cases there
may be some buildup of iron deposits that are lying on the surface, and
those can usually be removed by using a scouring powder like Ajax or Comet.
On rough-textured shells use a small stiff-bristle brush.  On smooth shells
use a soft cloth with a paste of scouring powder and water.  Of course this
may be difficult to do with a "mini shell", I realize.  For iron deposits
that have penetrated the surface and become incorporated into the actual
shell substance, there is only one way to remove the stain that I know of,
and that is to remove the part of the shell that contains the iron deposits.
That can be accomplished with a weak acid like vinegar or lemon juice.  I
don't recommend this for a glossy shell like a Cypraea or Oliva, because
even a short exposure to weak acid can dull the gloss.  However, on a
rough-textured or non-glossy shell, a few minutes' exposure to vinegar or
some other weak acid will remove a thin surface layer of shell, along with
any mineral deposits in that layer, without changing the appearance of the
shell noticeably.  Commercial rust removers might be used, but I believe
they are considerably more acidic than vinegar, and it is best to keep the
acidity as mild as possible.  Also, while the acidity could be tempered with
baking soda, completely neutralizing the acidity would render the solution
incapable of removing rust.  Rubbing the stained spot with a cloth dipped in
the acid solution is preferable, but if it is necessary to immerse the
shell, the glossy aperture can be protected from the acid by a coating of
petroleum jelly (Vaseline).
        Paul M.