The Capacitive, Courageous, Indomitable and Sinister Primal Diamonds are all new meta gems that give various new effects, from Spell Haste from the Sinister to the Capacitance effect from the Capacitive. There's also the Tyrannical Primal Diamond, which is PvP oriented but lacks the interesting proc effects of the other legendary gems. Most current primal meta gems just have standing effects like reducing snare and root duration or increased critical damage, so it's interesting to see these new proc based meta gems.
Since they're legendaries, I expect they come from the continuation of Wrathion's quest in 5.2, and we'll see how they relate to other benefits like the Crown of Heaven.
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.