I wonder if it's a form of verdigris that has this appearance because it's a thin layer of copper plating over zinc. It's showing in small places next to devices and also on the reverse. Maybe @BadThad has an answer.
Thats exactly why I question them. The zinc showing in the split plating and the zinc on the obverse are the same color and tone
Could this have been an end coin of a roll, obverse out, that was exposed to something while in storage?
No error, just stained or toned in that area. I would hit it with water and see if it comes off. It looks like an AU coin so no telling where it's been.
And if it doesn't? I have rinsed them with acetone and no change.. Usually to a darker tone and doesn't look natural. Maybe a simple nontoxic science experiment? I have found maybe five or six of these in the past 4 years.
Conceivable, but I'd expect that if it had been exposed to those elevated temperatures while also exposed to air, it would've darkened as well. Maybe not. I feel that experimental itch coming on. I think I've got the goods at this point to cobble together a fairly well-controlled heating box.
Possibly but the heat would have to have been very localized and controlled since the reverse and periphery are not affected. I've experimented with a mini blow torch many times in the past and this is not a color/pattern I could replicate, it's too even and monochromatic. Heat will usually product multiple colors. I'm still leaning toward a stain.
I was thinking along the same lines, but wondered if the striking process might have left the copper layer thinner toward the center. I doubt it; if anything, it should be thinner where it's stretched the most, and it seems like that would be around the devices...?
I find these from time to time. Most of them though are in bad shape, but the last one I found was decent. I hold on to them in case it is a plating issue. I keep them in a tiny envelope with my actual errors. Maybe someday I'll find out, but I think it's one of those cases that an expert would have to look at them in hand and there's nobody in my area.