Ever since Knuckles can remember, he has been the guardian of the Master Emerald on the Floating Island. He doesn't know for sure exactly why he's been handed that duty, but he accepts it with full dedication. He knows that it's the Emerald's power that keeps the island afloat, and he takes his job as guardian very seriously.
It wasn't until Doctor Robotnik crashed the Death Egg on the Island and convinced Knuckles that Sonic and Tails were after the Emeralds in Sonic 3 that things started to go wrong, and Knuckles entered the series of games. Robotnik knew that the Master Emerald had power, and he knew if he had it there would be no need for him to bother with the Chaos Emeralds. With the Master Emerald he could fuel his Death Egg forever.
In Sonic Adventure, Knuckles is awaken by the shattering of the Master Emerald where he sees Chaos, who quickly disappears. The Floating Island falls without the Emerald's power to hold it up, and Knuckles goes after the missing pieces of the Emerald to put it back together and restore the Floating Island in the sky. There is nothing to fear because Knuckles accomplishes his task, finding all the pieces, defeating Chaos, and restoring the Island in the sky, but the fact that something is always happening to the Emerald makes me truly wonder its power. We know that it holds the Island in the sky, and that it has enough power to fuel Robotnik's Death Egg, but what else can it do? I don't even think Knuckles knows the extent of its power.
Knuckles is transported to the past in Sonic Adventure, where he sees a female named Tikal, and he learns a little more about the Emerald's past. We see Tikal praying to the Emerald, begging it to take everyone to a safe place, as her father has started war with other tribes. The Emerald does nothing, though, and she says she understands.
In Tails' game, Tikal tells Tails something her grandmother taught her:
The keepers are the seven Chaos. Chaos is power enriched by the heart. The controller is the one who unifies the Emeralds.
It isn't until Big's game though that Tikal makes the connection that the Master Emerald is the unifier.
There's not a whole lot to be made out of that, other than that Tikal was either really hopeful, or that she somehow knew something about the Emerald that no one else did.
I think it's important that not a lot is known about the Master Emerald; even Knuckles doesn't know much about it. It's what makes his destiny interesting.
I'll probably be on this floating island forever, guarding the Master Emerald once again. I may not know the whole story behind this, but perhaps it's better that way. I'm at peace once more.