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".
I would just say that it toning. It's not very common but copper will oxidize to look white or silvery. I believe that @robec has a few in his registry. PCGS even recognizes the tone as natural if the surfaces are original. This is from PCGS. Yes Yes, copper coins can tone white or silver. PCGS graders look for original surfaces, and while toned coins can be graded, they must exhibit natural, attractive toning. Coins that appear unnatural or overly dark may not receive a high grade. Additionally, copper coins can change in appearance over time, which can affect their grading and market value
The heat theory sounds reasonable. U.S. Mint bronze is made of 95% copper and 5% tin and zinc. That's not say that the alloy is mixed perfectly in every cent. There could be a concentration of zinc in that spot.
This is subtle but it has an area that is toning lighter, maybe even eventually towards the silver side. And this has turned silvery on the reverse.