Jon did have the stone. But he was so startled by being caught by Sarah that he had forgotten. He looked down at it in his hand and was about to say "I wish", when suddenly Sarah grabbed it out of his hand.
"Were you going to throw this at me?" she asked.
"No."
"First you make everyone think that I'm a guy, and then you decide to stone me?"
"It's not like that. I wasn't."
She leaned in close and grabbed the front of the shirt that he still had on. "Listen to me, you asshole. If only dreams could come true, I'd wish that ..."
The stone, apparently, also granted wishes that began with "I'd wish" or "I'll wish", so the stone did grant Sarah's wish.
