As for gems, just do it. Enchant your gear. There's nothing flashing a red light of warning as an unenchanted tank. If you can't be arsed to enchant your gear, do the rest of us a favour and don't pretend to tank content.
That said. What should I enchant where?
Head: Get your reputation to revered with Earthen Ring and buy Arcanum of the Earthen Ring. 90 Stamina and 35 Dodge Rating is one of the most powerful enchants available to us.
Neck: Can't be enchanted.
Shoulders: Therazane reputation first to honoured and then to exalted. Exalted gives us Greater Inscription of Unbreakable Quarts. 75 stamina and 25 dodge rating is a very strong enchant.
Cloak: Protection. 250 armour is a pretty lackluster enchant, but it's the only one for us in this slot.
Chest: There are two stamina enchants here. I'll list the expensive one, but be warned that it's a lot more expensive than the secondary option. Greater Stamina. 75 stamina is a good enchant. A bigger health pool allows us to concentrate on mastery.
Bracers: Only one good enchant for this slot.Dodge. 50 dodge rating. We want to pad out that CTC. You shouldn't be so health starved that you want to apply the old WotLK stamina enchant here.
Weapon: If at all possible you should have a variety of weapons available, all with different enchants. I'll only list the obviously tanking oriented ones. Cheap one is Mending. Occasionally heals you for a small amount. The expensive one is Windwalk. With a close to 30% uptime 600 extra dodge rating and 15% increased movement speed is a fantastic enchant.
Shield: Mastery. 50 mastery rating is exactly what we want.
Boots: Mastery. 50 mastery rating is exactly what we want.
Legs: One cheap and one expensive option here. I'll list the expensive one. Charscale Leg Armor. 145 stamina and 55 agility is a huge increase in stats. Currently the agility is worth 35 dodge rating. With patch 4.2 it'll be worth nothing, so I hope it's changed into straight dodge rating.
Edit: Looked up MMO Champion. The PTR notes indicates that a new leg enchant is indeed incoming. 145 stamina and 55 dodge rating. That would be a net gain of 20 dodge rating then.
Belt: Belt Buckle. Gem with 40 mastery rating.
Hands: One expensive and one cheap option. I'll list the expensive one. Greater Mastery. 65 mastery rating.
The above are the default enchants. For specific jobs other enchants might be better, but if you're pulling those stunts you really didn't need to read this guide in the first place.