Found this in a box of assorted items. I think I got here somehow. It's graded by a "basement" grading company called NGS (Numismatic Grading Service). MS65 also!! Can anyone tell me why Lincoln has gone gray haired? I thought these coins were 96% Copper and 5% Tin and Zinc? How is it showing the Tin and Zinc? Is it Post Mint Damage? Why was it graded MS65? So many questions, so many presents to wrap!!!
I would call that damage. Maybe it wasn't quite that bad when it holdered and they thought it was toning.
Not a chemist, but I suspect it is toning of some kind, and whether it showed that badly or not when originally graded, many graders would not let toning alone lower a grade. Many people seek out unusual or "pretty" toning, and would consider it a bonus. Personally, I consider all toning a kind of environmental damage, since something in the environment had to chemically react with the metal on the coin's surface. As is often said of damaged coins, "It didn't come from the mint that way." That spot could be where someone, perhaps a Treasurer of the United States or other official, held the coin at one time and had something unusual on his thumb. Perhaps at some point someone pushed it into a coin holder or album slot.
Looks like heat damage. Zinc has a lower melting point than copper so it tends to rise to the surface.
Powder burns? Maybe it's residue from re-used acetone, or from some dipping crap that didn't rinse out all the way. 96% and 5% = uh oh.
Just toning, I reckon. Maybe heat related, as mentioned. Don't need to tell you that "NuGrade" is a Third World service with no reputation. "NGS" might as well stand for "Nobody Grading Service".