Adding to what BSM said, the "fingerprint" is in the ratios of isotopes of elements. Although all planets and asteroids (and meteorites) presumably originated at the beginning of the solar system, they contain different ratios of the isotopes of various elements.
A big reason for this is that earth has a molten core kept that way by radioactivity, whereas most asteroids are too small for this process. The molten core melts material, causing isotopes to fractionate. Lavas from different volcanoes are, to some degree, recognizable from their isotopic fingerprints (there was an article on this in Science News recently, if you want an accessible source). Also, radioactivity at the core might produce breakdown products from fissioning heavy elements...
To make a long story short, one can distinguish between a meteorite and a rock, simply on the basis of the isotopic fingerprint. A blade is another story.
I'm just guessing that Prof. Piaskowski can't distinguish meteoric iron in Keris blades either because a) he hasn't found a genuine one yet (this is a provenance question) and/or b) the meteoric iron is mixed with terrestrial iron and/or nickel in order to form one layer of the pamor. Since the earth is definitely not uniform in isotopic signatures, mixing metals could easily hide the partially extraterrestrial origin of some blade. A purely meteoric blade would be easy to identify, but one of mixed origin would be difficult.
For the whiskey and wine crowd on this board, this is analogous to the problem of identifying the parent materials in a blended whiskey or wine, based on taste.
And before Nechesh asks, I'm a professional ecologist. I don't deal with metal isotopic chemistry at all, but I'm familiar with isotopes of lighter elements.
Fearn
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