He decided to try letting Jon pick--completely unintentionally--what it would become. Besides, if he did it that way Jon would just think it was a badly interpreted wish.
Softly, the stranger whispered into Jon's rock, "I wish that in the near future, a wish made on this stone will be interpreted to change the stone into something else, under these conditions. First of all, whatever it turns into will still grant wishes by being touched, worn, used, or otherwise treated appropriately. Second, doing so will be inconvenient--either naturally, or because it has to be worn or used for a while, or because it has some side effect. Third, interpreting the wish that way doesn't seem like too much of a stretch. Fourth, once it's transformed, it can't be turned back."
The stranger carefully didn't say the next wish, because the stranger didn't want it to be wasted on something stupid, like turning into a cheeseburger when Jon wishes for food. But it would happen eventually.
The stranger wondered what it would be. Most wishes were for things like "I wish I was at school", which couldn't be done by turning the stone into something or "I wish for some money", where the stone could turn into money but carrying it isn't inconvenient.
On the other hand, if Jon wished for a bra, the stone would become one, and Jon would have to wear it to get wishes. It might require that he wear it for ten minutes before he can make wishes, or it might just magically make people want to wear it, or perhaps just having to wear a bra would be inconvenient enough. The stranger had no idea why Jon would wish for a bra, of course, but maybe Karyn would wish for one, or Jon would wish for a new car or a big penis or something....
