Meta Gems

The best gem for your meta socket depends on if you have Fleet Footed or not. For pure DPS based gems, the choices are;

Relentless Earthsiege Diamond
Chaotic Skyflare Diamond

I prefer the Relentless Earthsiege Diamond – if you take away the +0.25% crit that the agility on it gives you, you’re comparing +21 agility and +0.3% dodge to the Skyflare’s +0.21% crit.

If you don’t have the Fleet Footed talent, you may prefer to take this gem instead;

Swift Skyflare Diamond

The following gems may also be worth a look, though I haven’t seen any high ranked rogues bothering with them;

Potent Unstable Diamond
Persistent Earthsiege Diamond

Red Gems

Currently, the best overall stat for a red gem is Attack Power as your poisons/envenom damage receives more bonus damage through raw AP than agility.

Bright Cardinal Ruby or Bright Scarlet Ruby

An exception to the unwritten ‘rule’ that AP is best is the Fractured Cardinal Ruby. With a combat spec using Mace Specialization, gemming entirely with +ArP is supposed to absolutely flatten plate wearers, esoecially when combined with Grim Toll procs. I’d try it out, but I’d have to resocket and spend my last Arena Points of the season on an OH mace I might never use again….

Yellow/Orange Gems

Yellow gems give crit, hit and resilience. You need 164 hit to be capped against all classes except Night Elves (to guarantee a hit on a Night Elf you need 7% hit rather than 5%), if you are bellow this then it’s worth gemming for +hit. Resilience is a tricky subject – 700 is a nice amount to aim for, but unlike other stats it doesn’t particularly lose its value at certain levels. I wouldn’t take +resilience instead of +attack power, but personally I value it more than +crit for yellow sockets.

+Hit Gems
Rigid King’s Amber or Rigid Autumn’s Glow
Pristine Ametrine or Pristine Monarch Topaz

+Resilience Gems
Kharmaa’s Grace (Bought with 20 Stonekeeper’s Shards from Winter Grasp)
Empowered Ametrine or Empowered Monarch Topaz
Mystic King’s Amber or Mystic Autumn’s Glow

Blue/Purple Gems

To put it bluntly, blue is a horrid socket (the only useful ‘blue’ stat is stamina). If you are using epic gems you may wish to ignore blue sockets and gem them with red attack power gems instead. If you need to use a blue gem (perhaps for your meta gem), the previously mentioned green gems and the following purple gems aren’t too bad;

Steady Eye of Zul or Steady Forest Emerald
Vivid Eye of Zul or Vivid Forest Emerald
Balanced Dreadstone or Balanced Night’s Eye

Summary (Tl;dr)

Red = Attack Power
Yellow = Hit/Resilience (whichever isn’t high enough)
Blue = Meh

The Small Print

I lied – the best gem, of any colour, is this one. Unfortunately it’s only available to Jewel Crafters;

Bright Dragon’s Eye

(I wouldn’t leave Dalaran without them).