It's going to be tough because a lot of coins would fit that description, ancient or otherwise. I would suggest trying to post what photo you can - maybe we'll be able to see something there. Another option would be to do a paper trace etching - place a piece of paper over the coin and evenly color the paper with a pencil in an even back and forth motion revealing the surface of the coin - this would
work especially well if the coin
still has a varied surface and isn't worn flat.
Chances are it isn't iron. If it is iron it probably isn't an ancient coin (could be a button or a token). I'm betting that it will end up being bronze.
Could you describe the coin's circumstances? Like, if it was found buried in a
field in Yorkshire, we're going to think about that differently than if it was found on the sidewalk in Los Angeles.
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