Not to seem like a total n00b, but is this quarter missing a clad layer? I suspect that its simply an environmental effect, but I want to be sure. I haven't a scale or accurate calipers to measure with.
Put it between two ordinary quarters and check the diameter. Put it on a table next to an ordinary quarter and check the thickness. Put a pencil on the table, and put another one on top at a right angle, and adjust it until it is balanced. Then put an ordinary quarter on one end and yours on the other, to see if there is a significant difference in weight. If diameter is right, thicknesses are noticeably different, and yours is noticeably lighter, you may have a missing clad. Otherwise, definitely damage.
By all means, do the test as described above. I've got a bunch of quarters that look just like, or similar to yours. I used to artificially tone coins just for the fun of it. They'd often come out dark, like yours, but every once in a while they would end up with vibrant rainbow colors.
