Ember Primal Diamond

Not sure how useful this is with the new fixed MoP mana pools. I get an extra 6k mana out of this, which is barely 1 heal. I think Revitalizing Primal Diamond (SPR/+Crit Effect) will be a really good choice until the point where you want an INT meta (if that happens, it probably will eventually, maybe even already). The INT metas is what I go over the rest of the post.

Mana Tide regen is based off of Spirit as is Discipline Priest's Rapture now. Hymn of Hope from Priests and Druid Innervate are based on max mana but even then the extra mana that you get from those regen sources due to the extra 6k mana from this meta is extremely minimal.

Paladin's Divine Plea is regen based on max mana as well. It's 12% over 9 seconds, so normally you would gain 36k mana from it but with this meta you would gain ~36.7k. Let's say you get to use it 3 times over the course of a raid fight, you barely made 2k more mana than you would have without the meta.

Priest's Shadow Fiend is 3% mana per attack totaling 72k mana gained. Usually it can be used twice in a fight. This meta means an extra ~1.5k mana per use or 3k total per boss fight. Between Hymn of Hope, Shadow Fiend/Mindbender, a Priest's benefit from this is pretty decent imo. Hymn and Fiend can even be stacked to benefit from the increased mana pool buff Hymn of Hope gives (+15% max mana, which yields 7k more max mana with this meta) and the effect is compounded with this meta to make it even better. So I don't have much issue with this meta choice as a Priest.

It's no where near as powerful as it was before MoP though.

Depending on your crit chance and any class mechanics that have you crit often (Paladin's Holy Shock) may warrant the use of the Burning Primal Diamond (INT + 3% Increased Crit Effect).

Looking at it the same way I did Divine Plea, if you crit around 50 times in a fight (I think that's being conservative, especially in later tier gear) and you crit for a respectable 80k heal (also conservative), you would be getting almost 2.5k extra healing each crit if you used the Burning Primal Diamond. That could be over 100k extra healing and I truly feel that could be doubled or tripled easily without conservative numbers. In the end, an extra 300k healing also isn't a big deal just like an extra 2-3k mana from this meta, or let's be generous and say this meta could get you 5, 6, or even 10k mana due to Innervate, self used CD's, Hymn of Hope, it's still a very small amount in the big picture with either meta choice.