Gemming for raids.

As a rogue gemming is where alot of your stats come from, you may notice that some gear has alot higher damage stats but no hit rating but gem slots, this is where you must learn to find a comparison of point weighting, this term orginally coined AEP (Agility Equivilent Point) by Ming, love him or hate him, the guy did spawn the entire rogue theorycrafting system.

Moving on, the following stats are your most important:
Hit Rating (White damage makes up over 50% of your total damage)
Agility (Highest DPS stat as it provides AP/Crit/Dodge/Armour)
AP (This stat is usually given from raid buffs)
Crit (This is usually stacked from your agility, not a viable gem)
Expertise (No gems for this stat)
As such you want to have as much of these stats as you can, and in that order.

As such, you should always gem +8 or +10 hit rating until you are above 300 hit rating, once you exceed 307-316 hit rating, you will need to replace some gems with +agility/+hit rating combos (Orange Gems) as such, blue gems are the minimum and epic being the prefered.
However, i am aware that there are socket bonuses you may want to gain the benefit of, so as such, here is how you should and shall gem your gear.
Yellow: Pure +Hit Rating Gems, (Once hit cap is reached hybrid +Agility/+Hit Gems)
Red: Hybrid +Agility/+Hit Gems (Once Hit cap is reached Pure +Agility Gems)
Blue: Hybrid +Agility/+Stam Gems (These are usually kepted though an entire set for meta gem bonus and only two are needed, anymore, you are gimping your DPS)

The following gems shall not appear for any reason in any gear you have without someone laughing at you:
[Bold Living Ruby] / [Bold Crimson Spinel] (Strength Gems)
[Wicked Noble Topaz] / [Wicked Pyrestone] (Crit/AP Gems)
[Smooth Dawnstone] / [Smooth Lionseye] (Crit Gems)
[Jagged Talasite] / [Jagged Seaspray Emerald] (+Crit/Stam Gems)