It was the Slave Stone. It looked exactly like the stone Jon's grandfather had sent him, but it behaved very differently. It will make anyone who wishes on it be a slave to their wish, and anyone just picking it up will have to make a valid wish before they can even put it back down (though there is no time requirement, so the person could hold the stone for days, weeks or decades before making the wish).
The Slave Stone will only grant wishes that affect the wisher directly, except for wishes to learn or know something; anything else will simply be ignored by the stone. Once a wish is made, the wisher will find themselves unable to reverse the wish, unable to express a desire to reverse the wish, and unable to even communicate the fact that they made the wish. And finally, the Slave Stone will not alter other people's memories, and wishes made are reversible (if someone else somehow does the reversing).
For example, if Karyn had held the Slave Stone when she said, "I wish I had long blond hair and big boobs and maybe then I'd get some of the attention," she wouldn't be able to tell Jon that she wanted to reverse the wish even though he was right there and heard her. Everyone around her would notice that her hair was suddenly blond where it had been brown moments before, and if Jon then asked her, "Why did you make that wish?" she would deny making it even while in her mind she might be begging Jon to help her. Karyn would also never consent to have her hair cut shorter or even dye it a different color, no matter how much she would really want to. She could however make another wish to make her hair even longer, or to make her boobs even bigger, since neither of those would contradict her earlier wish.
