OK, one guy is saying 4%, another 20% and I saw someone on a different thread claiming 25% - is there a source anywhere on actual rate?
Anobix and I are essentially saying the same thing.
Short answer: WoWHead
lists each epic gem at roughly 4%. Add in the fact that there are 6 colors, and 4 * 6 = a 24% chance.
When you prospect some ore, it "rolls dice" for every gem you can create through prospecting from that ore.
For example, Titanium Ore has a 25% chance to prospect into each green-quality gem, a 4% chance to prospect into each blue-quality gem, a 4% chance to prospect into each epic-quality gem, and a 60% chance to prospect into Titanium Powder. That is to say, each gem gets its own dice roll to determine if it will appear in that prospect.
So for example, let's say we prospect 5 titanium ore.
Every single thing it can prospect into gets its own individual roll of a 100-sided die. So the game rolls a 100-sided die once to see if it lands in the range of 1-60. If it does, then Titanium Powder will appear in the loot table (It rolls again to determine just how much titanium powder, too). Then it will move on to the green gems. Let's say for example, that it will start with Bloodstones. It will roll the 100-sided die again to see if it lands in the range 1-25. Let's say it lands on 42, so Bloodstone will not appear in the prospect. It then repeats this for every green-quality gem, before moving on to blue-quality gems and rolling the 100-sided die again, but with the range being only 1-4. After that is done, it moves on to epic-quality gems, and rolls again to see if the die will land between 1-4 for each gem.
Thus, every epic gem has a 4% chance to be prospected, and in the long run, it will appear as a 24% chance to prospect into a random epic gem.
But there are other factors too, such as making sure you will always get something out of a prospect, or making sure you don't by some miracle of chance get every single gem you could get out of one prospect. Note: Take this with a grain of salt. This is purely my theory based on personal experience and data from databases such as WoWWiki and WoWHead. I have seen no post or other confirmation pointing towards this method as the true method for how the game determines prospects. Feel free to karma-bomb this post into oblivion if it proves inaccurate. Edited, Dec 29th 2009 1:41am by IDrownFish