If Jon was experienced in use of the box and stone, he would have wished that he couldn't lose the change, or even just disintegrated it. He wasn't all that experienced, but he had something of an idea anyway.
He got the bag out of his pocket and removed the stone, then made his wish.
"I wish to have the special power to see if an object can grant wishes, just by thinking about it."
Jon concentrated and suddenly the change and the stone all glowed purple. The stone was a bright purple; the coins were a less bright purple. A couple of them weren't glowing at all.
Experimentally Jon put down the coins and held one of the ones that didn't glow. "I wish for a slice of pizza." Nothing happened.
Then he picked up one of the ones that had glowed, and made the same wish. He got his pizza, and when he concentrated again, the coin didn't glow. Nor did it put pepperoni on the pizza when he made a second wish.
So the stone itself was a real magic item. The stone kept on granting wishes... and also, every item put in the box granted wishes, but only once.
Jon concentrated again to set aside the coins that glowed and put them all in the bag, while returning the rest to his pocket.
