I've noticed some mages are still stuck in the old ways of gemming! This is just a public service message saying that you should not be gemming for intellect and instead you should be gemming for Crit and Hit.
If you are familiar with stat weights and what they mean I'd have a look at this.
Now if you scroll down a bit, you'll see that the stat weights for a normally geared Fire Mage is
So what does this mean? For each point of a stat you gain that number in damage per second. Obviously, Intellect is going to win out because of how high it is rated when we're comparing it point for point with Crit and Haste. It has always been that way in the past, but now that MoP has introduced gems that have twice the amount for the secondary stats (everything but intellect) we now find ourselves in an odd position.
So, Brilliant gems have 160 intellect on them while Smooth Sun's Radiance has 320 Crit. That means according to these stats you gain 728 damage per second with the Brilliant gem in and 870 damage per second with the critical strike gem in making the Smooth Sun's Radiance the optimal choice by a pretty significant margin.
Smooth Sun's Radiance (+320 Crit) For yellow sockets
Potent Vermilion Onyx (+80 Intellect +160 Crit) For red sockets
Piercing Wild Jade (+160 Hit +160 Crit) For blue sockets
*If you're curious where this information came from, feel free to visit the source. simulationcraft.org