The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Hereditary Stone.
This stone worked exactly like the original stone, except that it grants wishes only by causing changes to the target's genetic heritage.
For example, if Karyn wished for huge boobs, at least one side of her parents' family would suddenly have a genetic certitude for women to grow very large breasts. If someone wished she was a hooker, Karyn would suddenly come from a long line of hookers and display hereditary physical and psychological traits (such as an addiction to sperm, perhaps) that made that career choice more likely if not inevitable.
