** Now updated for Cataclysm **
Leveling up Jewelcrafting can get very expensive, very quickly if you are buying materials from the Auction House.
If you don't need the income from selling metals, Mining is a great way to get the mats you need. If you're trying to level Jewelcrafting along with Mining, these charts may help with collecting gems through prospecting.
My Mining charts might also help you locate the ores you need.
Early and Outlands gems:
Wrath of the Lich King and Cataclysm gems:
How the charts work
Each gem has a chance to be mined from a node or prospected from ore. These values are gathered from wowhead.com and represent the percentage drop rate.
I've separated the mining and prospecting values for each gem, from each type of ore. This means you can see for example that you are more likely to get Star Rubies from prospecting Mithril than you are from Iron or Thorium.
Cataclysm gem drop rates are still only a few weeks old and therefore possibly not accurate due to the small sample size. I'll try to revisit the numbers in a few weeks to update them.
I hope you find these charts useful!