As it turned out, the stone he gave Jon looked exactly like Jon's stone, except that it had a thin white film covering it. It was the Obvious Stone. It worked just like the normal wishing stone, except people remembered things before the wish (just like all the other wished-up stones, except that Jon's new stone otherwise worked the same as the one he inherited). So, in other words, it didn't actually change reality, as any change was not seen as always having been that way.
But as said before, Jon wasn't the only one the mysterious figure had given one of his wished-up stones to. Who else in town received a stone, and how exactly did it work, other than not making changes to reality? The mysterious figure had left their stone with a note, of course, stating that the stone was magic and could grant wishes (leaving out the fact that they were defective in some way). He wanted people to use the stones right away.
