The Momentary Stone

The stone he put in its place looked exactly like Jon's stone, except that it was slightly translucent, making it look almost like plastic. It was the Momentary Stone. Any wish made with this stone would have a one-hour time limit on it, even if a different time limit was given as part of the wish. So, if Jon wished to be a girl, then he would change back to being himself an hour after he made the wish. Everything would last an hour, but nothing would last past an hour.