I can't remember if we've posted about the WoW Gem Finder or not, but the last gem list I posted about has gone missing, so if you've never heard of this one, it's new to you. WoW Gem Finder is a quick web tool that you can to quickly find exactly the gem you're looking for -- just choose the colors, attributes, and/or abilities you want from the checklist on the side, and it'll narrow down exactly the gem you want and where to get it. And all the gems link to Wowhead anyway, so even if the little description isn't enough for you, you can go searching for it elsewhere as well.
Pretty great resource for anyone (like me) just starting to pick up epic gear on their latest character and looking into where the gems are coming from. One thing players might still need help with is when to put which gems on which gear (most people wouldn't throw epic gems in gear you get at 61, I'd think, and I personally usually don't bother with anything but vendor gems until I get an actual epic), but that may be all outside the ken of this finder. As a tool to help you find exactly the gem you want, it's a good one.