The stone's origins

Upon arriving home Jon hid the stone back in the box he kept his porn mags in, and stuffed it under the bed. He spent the whole night in a kind of daze, thinking and rethinking wishes, imagining worst case scenarios and agonising over Karyn's mistake earlier in the day. As he lay his head down in his bed, his thoughts turned to the stone itself. Yes, his grandfather had found it, but where did it *come* from?