Chaotic Skyfire Diamond
(or CSD for short) is the best meta for warlocks (also boomkins that I know of I don't know about mages but suspect the same). What it does is increases critical damage by 3%. For normal crits this means the critical damage compared to a normal spell is 154.5% and if you have talents like Ruin (I believe boomkins have something similar) * which increase your critical damage further it goes from 200% to 209%.
150*1.03 and then for ruin that double dips somehow, we confirmed the behavior on beta and it was also noted on EJ as working as such.
This is strong enough to make it so that you would rather have a helmet with a meta socket rather than having spellstrike. T4 being the choice for warlocks.
CSD was added later in the game as an alternative to Relentless Earthstorm Diamond
, which does the same thing for melee. Originally, the crit damage bonus on Relentless Earthstorm Diamond would apply to both melee abilities and spell abilities before being split.