Jon thought he'd be clever by wishing his original stone back (it had nothing to do with memories, so he thought that the stone would grant the wish), but nothing came of it. Apparently, according to the stone, his wish contradicted a wish that was previously made with the Obvious Stone. Whoever stole his stone and replaced it with this one must have made that wish.
For the time being, it looked like he was stuck with the Obvious Stone, unless he could somehow get the other stone back. But the thief seemed to have covered all of his bases.
Jon sighed, but then counted himself lucky that he at least had this stone. Sure, it had one little drawback (although a lot of people in town might not call that "little"), but it still granted wishes, just like the one he got from his grandfather.
He stuffed the stone in his pocket and left his bedroom. He had already checked up on his sister and brother, but not his mom. How different was she in this new reality?