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[Phil Tomlinson]

Yes, I suppose someone could have substituted a fake gold coin by gold-plating some platinum or uranium, but either would be worth more than the gold, so why bother?
 

I guess most counterfeiters will try lead first.

Yes, that's what I thought.  And not to belabor the point, but lead is lighter than gold, and so my solution would hold.  Nevertheless, if the actual puzzle is that the fake could be lighter OR heavier, then my solution fails, and you have to go to the other method.