What is on this quarter?

It has bubbles in it, but acetone isn't really helping. @CircCam

 

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Tar? Looks like it dropped on a newly paved street on a hot day or something.

 

It honestly looks like something was spilled on it that reacted with the metal.

 

I'll just drop this in for consumption. My first take and the feel I still have is terminal toning left unaddressed for the better part of a century.

 

Encrusted carbon from a fire? I’ve seen this crusty black stuff before

 

Yeah, I like the "encrusted carbon" idea. You could either remove it using a sharpened bamboo stick or perhaps try some sodium hydroxide or potassium hydroxide (caution when working with strong bases).

 

Looks like damage from some substance that was dripped onto the coin.

 

Looks like it was ina holder and got too hot or in a fire. Burnt plastic maybe