It's just a quarter that has been left out in some bad soil. I have found 1000's of them.
Stick it in some lemon juice and scrub it with salt and you'll see that it is just another quarter. I tumble all my dirty clad to make it presentable/spendable in just that, lemon juice and salt.
It's worth 25 cents and if you use vending machines, you don't even need to clean it!