There are two questions to be answered when you're going to gem a socket.
The first one is; which gem is best for this particular color? Easy answer is; Solid Majestic Zircon. The other is; do I have to care about the socket bonus? Easy answer is; No
These questions need to be answered for any class and spec and gemming as a tank is actually rather easy, generally all you want is stamina, stamina and some more stamina. There are very few socket bonuses out there that are worth gemming anything else for, actually only 9 stamina socket bonus or more would be worth it. The only exceptions are if you are under defense cap as dk/paladin/warrior or if you need a certain color to activate your metagem. Your metagem should be Austere Earthsiege Diamond by the way, no matter what class you play (yes, more stamina). That means you'll need a red gem somewhere among all those blues to activate that meta. Some solve this by placing a Nightmare Tear into one socket, which counts as any and all gem colors.
I wrote about the reasons for stacking stamina in another post about warrior tanking, but this is true for every tank class;
"- You can never have too much stamina
- Any other stats are secondary to stamina, but it's nice with hit and expertise too of course.
- Even if you gain stamina on an item, it has to be proportionate to any amount of avoidance that you lose.
It's hard to say how much hp avoidance is worth, since it depends on how much hp and avoidance you already have, and what kind of mobs you're going to fight. But remember that hp is a definite way to survive incoming damage attacks while avoidance only gives a chance to survive incoming attacks. People call it "Effective" vs "ineffective health". Avoidance is ineffective because as long as you don't have 100% of it, there is always a chance it won't help you at all. Avoidance also only helps against physical attacks, whereas hp helps against any attacks (though death knights have talents that convert avoidance to magic avoidance I think)."
- Defender's Dreadstone (Parry/Stamina) (Not for druids)
- Guardian's Dreadstone (Expertise/Stamina)
- Shifting Dreadstone (Agility/Stamina) (Only for druids)
- Sovereign Dreadstone (Strength/Stamina) (Not for druids) (Not recommended)
- Vivid Eye of Zul (Hit/Stamina)
Any of these are acceptable tank-gems, but remember that stamina really is the best way to go (as of yet, this might change in Cataclysm).
Another tip is to start looking for cheap gems as soon as possible, since prices on gems can fluctuate wildly. If you don't know someone who can cut gems, or can cut them yourself, it's a good idea to look for gems on AH every now and then and stack them up in your bank for when you ding 80. A gem should never cost more than 200g, and they rarely cost less than 100g. Most servers seem to sell gems for around 100-130g, depending on demand.