The Self Stone

It was the Self Stone. This stone looked exactly like Jon's previous stone, but it has a difference. Whenever it is used on someone else, it affects the user instead. For instance, if you wish that someone else has red hair, it will give you red hair. A wish that someone become your slave will be interpreted as a wish that you become your own slave, which will probably just fail unless the stone figures out a way to do it. Mentioning other people without affecting them still works, like "I wish I was sitting next to Karyn". As is usual, wishes can't be reversed and anyone who hears the wish knows that things have changed.