I don't know that I'd get my hopes up too high on acetone even budging this. You can try it. This looks like the type that's going to take some muscle. If it doesn't begin to dislodge in 10-20 minutes, give it an overnight soak in a glass jar. You want glass, as that won't react with the acetone. Give the lid a tightening so the acetone doesn't evaporate. You might luck out, but if you don't, you're going to have to get in there with the toothpicks and try to dislodge it. If I'm recognizing this type of adhesive right, it's a stubborn one.
At least one of the photos makes the stuff look METALLIC. I have zero idea if that’s just a lighting thing or what it is. If it looks metallic in the hand, whoa, just whoa. It’s gonna be a BEAR! Besides, everyone knows that metal like that should only be used to “create” “full head” Standers, smooth Morgan cheeks, and FSB Mercs, right? Hey, who said that?
You saying it’s not metallic, or that it’s a metallic-looking glue? If there is something like that, I do have a trophy that needs a little repair.
Are you better thinking of it like a plastic? That's really how it sets. That's also why acetone isn't going to dissolve it. We call it a "glue," but it may be a plastic compound. But at some point it's malleable, and can take impressions. Then it hardens and, you just about need a crowbar to get it off.
I haven't any idea. I know I've seen quite a few of these, mostly on cents. I know it's malleable at some point because it takes an impression before it hardens like a rock on the coin. Representative of quite a few cents I've seen is an impression of the reverse stuck onto the obverse. It looks exactly like it's part of the coin. If this is that material--and, I've seen it scattered, just like this, on coins, too--he's going to need a knife to get it off, if not, an industrial strength acid. Get this, though, if you get anything. The "glue" does look like a part of the coin, just like in this example.
After reading all of the comments, had to take a second look at this coin and I have decided not to do anything. If I tried to chip away at it I would totally destroy the coin, I appreciate everyone's information. THANK YOU!