... bit of strangely-colored dust.
"I wish I knew why there's suddenly this little bit of purple dust on my white sheets!"
And then he knew. He could sprinkle the dust on the stone and alter it. It would trade off time and space. The stone's magic would now affect the whole world--but the world is big, and it takes time, so he'd have to wait 15 minutes before making another wish. Trading off space for time.
Even if he just changed something small, the magic would have to travel everywhere see that nothing's changed, so he still couldn't make another wish for 15 minutes. The only way to make a wish without having to wait for the next one is to wish that it doesn't change reality, or to make a wish which inherently doesn't change reality, like wishing to know how to do something.
Jon sprinkled the purple dust on the stone, which glowed. He thought that it was a good idea. After all, how often would he really need to make two wishes within 15 minutes?
