Hey, i just asked Nik from MMO-Champion and found out that when you get the "legendary" gem, and socket it into a weapon, you can't remove it. Its there forever. And by the look of the answer i got from him, you can only get 1 gem.
To be fair, there's no way of knowing for sure right now that you can only get one gem (no one has gotten that far in the quest line afaik). Not to mention nothing is final. It seems like a poor design on blizzard's part to only let you get one, when there are heroic and normal versions of the sha-touched weapons.
It seems incredibly unlikely to me that Blizzard wouldn't make the gem replaceable or removable when there are 3 tiers of weapons that have the socket for them (LFR, normal, heroic). It seems similar to the faction-based rings they've had in the past for some dungeon tiers, and the lesson they learned long ago that they needed to make them exchangeable for cases where players end up changing specs (e.g. your guild needs a holy paladin now rather than ret and you're changing specs, and you need the int ring instead of str). I really expect that they will have a similar path for the legendary gem so that players who get better weapons or change their spec can continue to make use of the gem.
In general, Blizzard has moved away from punishing people with permanent consequences, and locking the gem permanently to the first socket it goes into really doesn't seem in line with the way they have been making the game for several years. I could quite easily be wrong about this, but to me it would be shocking to see the gem insertion as a permanent decision.
What Angriff said. Its still in beta and anything can still happen. And i agree that they should make it the way they did in ICC, that you can replace your gem or get a new one. For example put in a unique-equipped on the gem, so you can only have one on or something