Today I took my quarter to the “ premier” coin shop in my area and showed it to them. They were intrigued and put it in their spectrograph to get an analysis of the composition of the coin. A quarter is 75% copper and 25% nickel but this one is 87% copper and 10% nickel. The difference would be the tar like material that is on it. Then they weighed it and it weighs slightly less than a quarter would, again including whatever is on it. They believe it is indeed a mint error and they urged me to hold onto it and try and find a safe way to clean it.
Your “premier coin shop” was trying to get rid of you. If they felt it was worth something they would have made an offer. Damaged.
That, and/or they don't know what they're doing. A quarter is 91.67% copper, 8.33% nickel; it has two layers of 75% copper/25% nickel bound to a core of pure copper. At least until the outer layers are corroded away. Which, coincidentally (not), leaves it weighing less than a normal quarter. If they "measured" it at 87% copper and 10% nickel, what did they say composed the remaining 3%? Inspiration?
Sorry to burst your bubble, soak it in some distilled water for a couple weeks, follow it with a few days in acetone. See if you can get some of the corrosion off of it.
The coin, in the areas not covered by the tar, appears to have been subjected to a corrosive. Depending on the corrosive it could have removed some of the nickel resulting in an elevated copper reading.
