Protect his ownership of the slate.

It occured to John, that he could loose the slate, or somone could take it from him.

He tapped the emerald "Everyone besides me ignores the slate and anything I do with it unless I intentionally call their attention to it."

He thought for a moment and tapped the emerald again "Should I loose the slate, or if it leaves my possession for any length of time, it will always return to my possession."

He tossed the slate out his window and into the neighboring wood-lot. He waited patiently for several minutes and with a bump, the slate reappeared in his pocket.