You may have realized I am researching gems and gem sockets for the Solo MMO Player. I'm not ready yet to write a When, Where, What to Gem summary article but I will post what I discover here for your use until I get the Big Picture written.
The WOW Gem Finder looks like a very useful little tool as you can search for abilities or colors, and everything links to WOWhead so you can look up more information on their extensive database once you know what you are looking for. It is not up to date however as it doesn't include the changes in the gemming system that the Wrath of the Lich King expansion brought to the game.
There is a list of gems broken down into four categories of red, blue, yellow and meta on the Allakhazam site (i.e., it is not up to date either). Click on a category and a list of all those type gems opens with the effect shown. This isn't searchable and I don't think it is nearly as useful as the WOW Gem Finder.
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