If you are looking for large quantities quickly, then AH is your best bet. Gems don't appear very often when Mining (although the drop rate appears to have grown since the 2.3 patch....but that might just be me).
If you are willing to spend time, then listen to Cochu. In addition, I've had good luck with Iron in Arathi. Run a loop around it, and you can get quite a bit. Badlands also worked really well.
Good luck.