When you gem, what do you gem for? Do you gem Stamina as a tank only, or attack power for all melee, haste for all healers, or spell power for caster DPS.
Until recently (4 days ago) I was under the impression you had to match the gem to the socket color, whether in a solo color or a mixed color, I believed that you could not gem that socket otherwise.
I have recently through accident while re-gemming a socket found that anything would fit, and that the socket color only required a match to get that little socket bonus. Now you still need to take your meta into account as it has requirements as well to be active. The problem I'm sure for most is what to gem, for what role, and what outcome it gives. For example attack power is a static boost to pure DPS, however agility which provides attack power also gives crit and some armor, so do you add a 40 attack power (epic) gem, or the 20 agility variant. I myself having a melee Off spec to my healing Main spec, am finding it a challenge to gem "correctly" for everything that happens.
Sometimes when I heal I wish I had more mana re-gen, other fights more haste, and have now started to weight up mana re-gen vs mana pool, haste vs mana re-gen... What I need is to make a standard gem set, from random heroics, all the way to two healing TOC10 and then further to progression ICC. I want more haste as I need it, I want mana re-gen as some fights burn me down when I'm healing aggressively to keep everyone up, I want a larger mana pool so I can heal longer, Haste to heal faster. Usually spell power is what a healer wants as that allows them to heal for more but since my trinket/totem/ring, all randomly proc to give increase spell power as I heal Im not gemming for them when I can avoid it.
As it stands I'm looking into 10 haste + 5 mana per 5 epics, to slot, I want around 500 mana per 5 while casting, once I have that with my totem down, It's all Intellect and haste till I find my middle ground.
My advice is look it up on different sites, and then make your own choice, I want the mana re-gen, not much else matters other than haste, so there my goals. My enhancement spec is gemmed for hit/attack power/agility I've added everything, but being a leather worker I can lay off Attack power gems and focus on hit/haste/expertise, etc.
Take into account everything you can, and gem to what you want, if it's not the best it can be, then tweak it, if you think you can get more, change it, but start off where you want, with what works for you. Start with blue gems, there cheaper and easier to get, save the epics until your certain thats what you want to have.
Egamad of Caelestrasz