The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Dilemma Stone. The effects of this stone vary wildly depending on the scale of each wish requested of it:
A small wish (such as transforming a small object or animal, or altering a small part of someone or something larger) would make it so that everyone remembered reality only as it was after the wish was granted, whether they heard it or not. Even the wish maker themself would forget they made the wish in the first place.
However larger wishes were not safe either. A wish on a larger scale would make no mental changes at all "“ not even to the memories of those who don't hear it...
