Direct from tankspot.com:
Meta gems
1. Austere Earthsiege Diamond (629p) Rare
The boss tanking meta gem. Scales directly with the quality of your gear. No reason not to have this as your primary meta gem.
2. Eternal Earthsiege Diamond (384p) Rare
This meta gem is kind of a joker. The benefit from using it scales with how much Strength, Block Value, and Block Chance you have. Certainly the meta gem to use if you have a dedicated "TPS" tanking hat. For non-raid tanking where mobs don't hit so hard, this meta gem will most likely be one offering the most damage reduction.
I'm seeing a lot of people who are all about the Eternal - rather than the Austere... (Counter to Tankspot's opinion)
Which is it? I'm not yet in Naxx, I could use more STA. I have about 22.5k HP, I'm something like 545 Def at the moment. I have yet to buy badge gear.
I have both in my bank - and nothing in my Titansteel Helm.