Gonna apologize in advance for ranting here, but I'm sure there's more people who feel the same.
So at first I was happy I upgraded my normal T14 helm to 504, thinking the legendary meta gem would fit in it. Apparently the tooltip is wrong, you need to have an actual Throne of the Thunder helm to use it.
I guess holy paladins will need to wait until 5.3 to use the gem, such a giant slap in the face if you ask me.... :S Really can't see why it wouldn't be allowed in any other PvE item.
/rant over
Edit: apparently it does work on the heroic version of the T14 helm, but yeah good luck convincing my guild to go back and do sha of fear heroic.