What is a quarter with only tails worth?

You either have one of the two pieces used to make a "Magician's Coin" novelty item (worthless). Or you have a pretty rare minting error caused when two coin blanks enter the coin press at the same time (worth $50+ to an interested error collector). If the weight of your coin is significantly less than 5.67 grams, then I would say you have the novelty item. If your coin weighs right, then I suggest you visit the http://www.anacs.com website, check out their show schedule, and take the coin to one of the shows they will be attending. They will give you a free professional opinion, and will also accept your coin for certification right there. Good luck! Dan