Have you tried looking the coin with very bright 'oblique' (from the side) lighting in a darkened room? Sometimes that bring out shadows of features from which you might be able to get a clue to the coin's identity.
Recently used this technique with a really toasted 1600's French coin. Under normal lighting there were no discernible features. With the shadow details, a forum member positively identified the coin. It is also very important to provide the diameter of the coin to help in positive identification.
I used a high-intensity LED flashlight to get the shadow image off the coin photo I've included in this response.
Good luck!
--Bert
See:
http://metaldetectingforum.com/showthread.php?p=2754400#post2754400