Hi, Ross and the rest of you!
An old book I have, by Walter Freeman Webb, says to dip the shellsin hot muriatic acid (yuck, that's dangerous) and then plunce them into ice cold water. Needless to say, I have never tried it, so I don't know if it works or not. But I do have some of those "polished" shells, which I obtained a long time ago from curio shops.
Cheers,
Linda