This stone, in particular, was the Amnesia Triplicate Stone, which multiplied whatever was wished for by three, while also making the wisher forget that he or she had wished for anything and think that the new reality was normal.
So, if someone wished for a million dollars, they would instead receive three million dollars, but would forget that they had made the wish and would think that it was normal for them to have three million dollars.
Anyone else, though, who was within earshot of the wish would remember the previous reality and know that the wisher made a wish.
