I have cleared basicly all HC TBC dungeons to test the different sockets, since wowhead gives an impression that some weird combos still exist. The only two that I ever found to give more than 2+ main stat was 5 int/6 stam (Black Morass) and 5 str/6 stam (The Shattered Halls). Drop chances are pretty random, some times I got 2 gems from 1 run and other times I got 0 in 10+ runs.
Remember these HC dungeons only reset once daily, so if you plan to farm them, you have to switch between chars that are able to enter and solo these dungeons. Around 36+ was fine for me.