My player was searching through a random pile of gravestone fragments and because he scored a nat 20 on his perception, I didn't want to tell him he found nothing. So on the fly I gave him a faintly magical stone with a single rune carved into it. It's the size of a marble, and perfectly cubed. It weighs nothing, and has no useful effects by itself. I had the idea that if my players find a lot of these, always by scoring high in perception, then they'll do something neat.

So what should that "something neat" be? :P The stones look exactly like role playing dice, but only have one face carved.