The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Morphing Stone. This stone could not grant wishes. However, when someone touched it and made a wish, that person morphed into whatever the subject of the wish was, for six hours.
For example, if Karyn says "I wish I had a dress," the subject of that wish is dress and the stone would cause her to morph into a dress. And she would stay that way for six hours, before turning back to normal.
Sometimes the stone would also cause the "wisher" to morph into something that is associated with the subject of the wish. Like if Jon says "I wish I was at school," the stone may not turn him into a school, but rather something associated with a school, like a school book, or a desk, or maybe even a duplicate of one of the teachers.
