General rule for gems for a Tank.
Red Socket = Dodge/Stam, Expertise/Stam or Expertise (only if you need it BAD).
Yellow Socket = Hit/Stam or Def/Stam.
Blue Socket = Stam of course.
Only gem for Expertise to get to 26, if needed, it is a waste to gem for Expertise beyond 26.
If your Helm has a +12 Stam Socket bonus, ALWAYS gem for the socket bonus. You will get 10 Hit/Def/Dodge/Expertise for the cost of only 3 Stamina.
To Activate your Metagem. Put a Dodge/Stam or an Expertise/Stam gem in your shoulders.