The stone he put in its place looked almost exactly like Jon's stone, but it wasn't. It was the Solo Stone.
It worked in exactly the same way as the previous stone - except this stone's wishes are reversible, and each time a wish is granted the stone's previous wish is dispelled automatically. As such, the Solo Stone may only grant one wish at a time.
What is more, any wish made on the Solo Stone becomes permanent until a subsequent wish is made, regardless of the wisher's specifications. If a wish by its nature cannot be made permanent then the Solo Stone simply will not grant it, instead holding onto the wish it currently holds active instead.
