Hello whitesid,
If the picture were clearer, it might be easier to determine. However, from what I see it could be any of several things.
I agree with BIG61AL that it could be a 'cud' (a certain kind of cud). But it could also be other things. I could just be a 'bubble' which was produced during the plating process. It could also due to a die defect. It appears to be interesting, but still, probably common (a minor error) and not worth much. It may be best to simply keep it as a conversation piece.
Regards,
JDSCOIN