One of the players is "in the Russian mob" ... and his typical M.O. is to shoot first and not bother with questions. After the first game, he had two black stones, and I said NOTHING about them. Someone else did something epic-nice and I gave them a white stone. Eyes widened, and then people started looking at the mobster. Every once in awhile, I would say, "That shot is a hit ... or you could miss to get rid of one of those black stones." That's as much hint as I have given them on how I run them. They have asked about using them like Fate points in certain situations. "Can I use this white stone to do _________?" and that is how they've learned the rest of what they "know." No one has accumulated more than two of them at once and no one has tried to used more than one of them at once. The folks who have collected black ones have not bothered to see what they can do, either - they have all sent me e-mails after the game on various "atonements" that they wanted to do to get rid of them.
It has been a hilarious meta-game addition -- at least for ME as the GM. :-)
I don't know if any of them read this board, so I want to keep the system secret, for now. But it's an easy concept to adapt in any way you see fit. I originally had a more "Jedi Knight" idea about it, but it has turned the party (even the mobster) into crusaders. On the plus side, it has made for good harmony within the party (which can sometimes be tricky), but I think it has made the world slightly less interesting because the players have started quickly labeling everything as "black and white." "Winter Court = Black." Well, no ... not really, they just have a different agenda. "US Government = Black." Wait, they did come in and clean up the airport you left smoking ... that doesn't mean ... "Red Court = Black." Okay, yes, yes, those are actually bad guys. So, I need to introduce some sort of "gray" stones, maybe ... I dunno ... "balance stones" or something.