The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Echo Stone. It worked just like the normal stone, except that the second person to use the stone was forced to make the same wish that the previous wisher made. But once a wish is "echoed", the next wish is whatever the wisher wants it to be. But then after that wish, the next wish is again an "echo" of the previous one.
So, for example, if Karyn made the first wish and Jon made the second wish, then his wish would be a repeat of Karyn's wish. The third wish is original. But the fourth wish is a repeat of the third wish. And so on.
