What would cause a black state quarter

I was searching through some quarters and came across this what would cause this? Thanks for any input

 

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Dirt, mud, grime, oil, burnt debris and just about anything coins touched, dropped in, etc. I find plenty of quarters like this, I buy snickers with them from the vending machine.

 

Might have been a metal detecting find. I've found quarters in the ground that look like that.