Not sure I would want to restore it unless it goes so black you can't see anything on it-- even then I would clean it only with expert help/advice. I would LOVE to find a coin like that! It's a Spanish Colonial silver cob coin (Peru mint I think) from 1600s to early 1700s.....I think the 6 there denotes a 1600's date.
Thanks for the info... I guess I was panicking a bit because it changed color from a matte grey to black at an alarming rate! When it was first dug the area that looks clean looked just like the photo, and still does, but the area that turned black had a different finish to it, almost like a dull pewter. So I was interested if anybody knew how to get it back to the way it looked in the first few hours above ground or maybe to stop this process from happening.
Guess I'll need to research further on possible dates and why the thing would have a big hole in it..Fun and fascinating!
I'm surprised it turned black that quickly. silver does turn black in natural sunlight over time. But I wouldn't worry to much, silver is pretty much bomb proof.