Well, not always. I've dug 10K items that were VERY tarnished. The gold does not tarnish, but whatever other metal is used does.
Check out my post:
http://www.metaldetectingforum.com/showthread.php?t=7686
However, if this were the case with the item in question, it wouldn't polish up as easily as it did. Plus, it's hard to imagine someone making a copy of a coin out of real gold and not marking the karat level.
I agree Z118... I have found several old 10k and even 14k rings that bleed out whatever metal they are mixed with and I have 2 10k rings that are black and tarnished... I'm guessing what's bleeding out is copper because its usually green..