The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Exchange Stone.
This stone granted wishes, like the original stone, except that it granted them only by exchange.
For example, if Karyn wished for a car, someone else would no longer have a car, and the reasons for that could produce other unexpected consequences. If she wished her parents would buy her a car, she might end up with different parents...ones who could afford to buy her a car. If she wished she could afford to buy a car herself, she might find herself with an after-school job someone else had before, or someone else's inheritance.
If a wish was made that was not specific to a group of people, for example, if Karyn wished that everyone had a nice car, the stone would react unexpectedly against the wisher.
