Oh something vague which means they haven't figured out what to do with them yet, but maybe if they have time later they'll code something in for them to do.
Just kidding! It actually kind of sounds like if you have good karma positive stuff will happen around you (positive karma creatures will randomly spawn around you?) and if you have negative the reverse will happen. Or maybe healers or priests of mondain will bow to gem holders...thats all I got. It sounds interesting though!
What they do is, in terms of "power" or "effect" is nothing at all, but to me, on a personal level, what they do is probably one of the coolest things they've put into the game to date.
If you double-click the shard, based on your karma/fame (not sure if it's one, the other, or both), you get to "see" the world within your gem, and see how it's doing. At "trustworthy," mine describes a world where human, elf, orc, and gargoyle have come together, living in harmony, their problems overcome, and for one brief moment there's a sensation of pure joy or something like that.
It's a very nice, very interesting way to handle the shards.
Of course, on the other hand, these shards also mean that Lord British is dead, since he took them with him into the Abyss, and years later, the Shadowlords are running around with them. That can't be a good sign for ol' LB's health.
And yeah... were it me, I'd want everyone who attended to have a shard. That's not how Mythic decided to handle it -- and from a differing perspective, I can understand why they did it that way too. It's a hard line to walk. I quite literally lucked into mine... and will never, ever, ever part with it, not for all the gold in Ultima there is. It's a very special, very RP-centric object that is really cool from that perspective. Sure, some may sell theirs for a couple hundred million or so gold... hopefully it will (as one of my friends reminded me I should hope for) end up in the hands of someone who actually cares what a shard of the gem of immortality really is.