Gemming

Gems

Red Sockets (+40 Intellect):   Brilliant Inferno Ruby

Blue Sockets (+20 Intellect +20 Spirit):  Purified Demonseye

Yellow Sockets (+20 Intellect +20 Haste):  Reckless Ember Topaz

Meta Socket (54 Intellect and +2% Maximum Mana, Requires at least 2 Yellow Gems):  Ember Shadowspirit Diamond

*If you are a Jewelcrafter, then you would use the Brilliant Chimera's Eye x3*

Depending on the socket bonus, you can consider using straight Intellect gems.  If the socket bonus gives any Spirit or Intellect, it is recommended that you try to get it.  If the socket bonus gives Haste, it is more of a personal preference if you get it or not.  

Why do you gem this way?

In Cataclysm, we still use Intellect as a primary stat.  Instead of just stacking it, we do add other stats such as Haste and Spirit.  Because our mana pool is so large, we get plenty of Crit Chance from the Intellect that our gems and gear give us.  Crit Chance is a little less important because it no longer gives us mana back.  We now have to rely on Spirit as our mana regen.  As far as haste goes, it was scaled down back in 4.0.  Because of this, we do partake in some haste as it comes our way.  Intellect gives us spell power as well as a base stat.  Since having strong direct heals is important for a paladin, we continue to use this gem more than others.