JS> > Any sample of a material will contain not only the base atoms but
 JS> > isotopes of the elements as well.  So, if you take your basic
 JS> > stainless steel ashtray and analyze it, you'll find iron (carbon,
 JS> > chromium, et al) and isotopes of iron (and the others, of course)
 JS> > consistant with their occurance in nature on the world on which
 JS> > that ashtray was made.
 JS> So what you are saying is, even if the "alien ashtray" were made of
 JS> similar metals found here on earth, we could tell that it was
 JS> "unearthly" in origin?
 We could not be completely sure.....but when have we ever been?
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