Which Gems should I use as a tank?

I'll try to be short and precise here, but let me say something clearly first: equipping a tank is always about a balance of stats. It is never about all this or all that. For specific encounters one may swap a bunch of pieces or even prepare a full kit maxed out for one aspect, but for your bread and butter tanking, you do want a good balance.

I'll list the gems by socket colour and include the non-epic choice for budget gemming.

Please note:

1. You don't _have_ to match socket color, you'll only loose the socket bonus. However, the socket bonus can be qutie nice. You should definitely get each +sta socket bonus.
2. Don't forget to get an [Eternal Belt Buckle] for that extra gem slot.

Blue Sockets are the easiest to fill. All tanks need stamina to build up their effective health.
[Solid Majestic Zircon] (30 sta) or [Solid Sky Sapphire] (24 sta).

Red Sockets are our second choice. Dodge (or Parry) plus Stamina improves Avoidance and Effective health at the same time without wasting any budget. Druid's benefit more from straight agility and can not parry, so avoid the Regal and Defender's cuts if your tank on all fours.
[Regal Dreadstone] (10 dodge+15 sta) or [Regal Twilight Opal] (8 dodge+8 sta).
[Defender's Dreadstone] (10 parry+15 sta) or [Defender's Twilight Opal] (8 parry + 12 sta).
[Shifting Dreadstone] (10 agi+15 sta) is the druid's choice.
[Delicate Cardinal Ruby] (20 agi) is a good alternative, especially for shared cat/bear gear pieces.

Yellow Sockets are a bit of a bother. Tanks usually do not need to gem for hit, expertise or crit, so we need to find a good use for those.
[Enduring Eye of Zul] (10 def+15 sta). Great for all plate tanks.
[Stalwart Monarch Topaz] As the plate tank budget alternative.
[Nightmare Tear] +10 all stats or [Enchanted Tear] +6 all stats are the bear's first choice for yellow sockets. I'll still need to see what to take next if needed.

Meta Gems are very easy:
[Austere Earthsiege Diamond] (32 sta + 2% armor) is the smart choice for effective health.
[Eternal Earthsiege Diamond] (21 def + 5% SBV) is a great choice for shield tanks running heroics or T7 content, or of course for use in a block set.

Of course the list above won't make it into the next expansion, but I hope it will be of some use until then.