Also, note that the gems from vendors (at least on my server) are hardly any cheaper than the uncommon gems from the AH, especially if you buy the jewel and get a friend to cut them for you. More bonus for the same price = good.
Also, you do NOT have to match the gem to the socket. If you would rather put +7dmg red gems in an item that has a blue and yellow socket, go ahead - only thing you lose is the socket bonus, which (especially after you start getting the rare gems) is hardly worth keeping if you favor a certain stat.