I just finished the first part of the legendary quest and received the Legendary Gem. Being the first of many on my server i wanted to test out the gem and find out a bit about it. I socketed the gem thought "How would I get another one of these if I were to get an upgraded weapon. I searched for a while on the forums, wowhead and some other fan sites and only found information about being able to buy another gem for a price unknown to many. I quickly found out that Wrathion offered the service to repurchase more gems for a hefty sum of 10k gold!
My problem is that upon completing the quest the game never gives information about how to receive more gems for upgrades OR the large sum of gold required for another gem. So for people like me who are some of the first test out these gems without much background information that the game or other related sites fail to give us, and end up having to spend so much on something we weren't aware of.