Karyn always loved Jon but even by flashing him with signals, he never responded to her advances. So, she start to think, if he don't want her, it's because she's not the kind of girl he likes. And the better way to be that, is with the wishing stone.
So Karyn take the stone and makes her wish.
"I wish I had the body of the perfect girl according to Jon." She said.
But as the stone is the carnal stone, the wish is corrupted as the perfect girl according to Jon's deepest fantasy.