First thing is first. If you dont have money to spend, I dont know that I would level jewelcrafting until you do. You will want to be at least level 75 if you are going to power level it. That way you can get to the real meat and potatoes.
It can be quite expensive depending on your server, so unless you are keen to hours of farming, it may not be the best thing to do.
This guide is meant for making money once it is already leveled. You have to spend gold to make gold.
So, once you get to the point that you can do the JC Dailies....do them. Even do the Northrend dailies before you are able to do the Cata daily. Many of the gems are never on the auction house, so you can pretty much make your own market for them. They are needed for leveling characters.
Now, I am going to go over some of the better selling gems. Obviously everyone will tell you that the red ones sell best. And that is the truth for the most part. Not all gems are created equal.
On my server in particular, yellow, green, and blue gems are not very profitable. Blue and Yellow seem to sell the least in my experience.
When you are ready to get that first JC pattern, your best bet is probably the Brilliant Inferno Ruby cut. I sell more of those than any other gem. You may be tempted to go for another red cut next....DONT. Unless you have a large number of stockpiled reds, I would wait.
Next, I would look at the red and purple cuts on Undermine Journal. Pick the one who best combines sell price and volume. For me, it is Reckless Ember Topaz.
After you have picked up either an orange or purple cut, spend the next on another red. Either Delicate or Bold. Then an orange or purple. Then the last of the three reds I listed.
I would take red or purple cuts for the most part until you start to find more profitable green, blue or yellows. I do sell quite a few Puissant Dream Emeralds, however they onyl usually go between 15-25 gold per gem depending on the day.
Dont forget Meta Gems. Unfotunately on my server, they dont sell in much volume, and the often times go for less than an inferno ruby.
Now, what do you do with your uncommon gems? With the upcoming changes in 4.2, vendoring will be out of the question. Zephyrite will probably still unfortunately vendored unless they come out with something to use them on.
If you have an alchemist, I transmute most of my Carnelians into Inferno Rubys. There is also the option of Meta Gems, or making Carnelian Spikes to disenchant into Greater Celestial Essences.
After stocking up on a decent number of green and purple gems for the dailies, I generally turn anything I can into Jewelry.
Once I have turned those uncommons into Jewelry, I post the rare procs on the AH for 100-300 gold each and disenchant the rest.
Obviously, having more professions will give you a better chance at making gold.
Nothing revolutionary here, but that is how I do it.