I'm sorry, I don't know too much about errors, but I think this is called a greased die error, and occurs when grease fills the die in which the coins are struck. From what I know, they are common and not worth much unless very large portions of details are missing, or if portions of detail are missing that makes the coin attractive in some way. I hope this helps!
If I've never seen something strange like that on a coin, my first and only thought would be to keep it. How many quarters have you seen in your life, probably thousands. How many quarters have you seen like the one you have? 1, maaaaybe 2 or 3. Keep it, save it, cherish the faulty die used on it.