The following is taken from Wikipedia.
Quote:
a mint stamp is a stamp which is in its original state of issue, is unused, has never been mounted and has full gum, if issued with gum.The term applies equally to postage stamps and revenue stamps.
In practice, the term is used within philately to refer to any stamp that appears to be unused, including those without gum, or previously hinged.
Unlike other collectibles, a mint stamp may be in poor condition but still be regarded as being in mint condition as long as it is apparently unused.
http://en.wikipedia.org/wiki/Mint_stampSo I think your stamp is still mint but maybe only F-VF. Instead of XF.