With the help of a reader, who provided me with the final missing gems, I finally managed to get my jewelcrafting skill up to 300. At this skill you can go to the jewelcrafting grand master and buy designs for gemcutting from him. So this is how gemcutting works in the Burning Crusade:
First you need to do some mining, typically for fel iron in Hellfire Peninsula. With the prospecting skill you get with jewelcrafting you then need to destroy the fel iron ores in the hope of finding cuttable gems. Up to now for me about half of my attempts gave me a gem, the other half just left me with worthless fel iron dust. The gems I found were for example deep peridot or azure moonstone.
For the azure moonstone I have two possible "cuts", either solid or sparkling. The solid azure moonstone gives +9 stamina, the sparkling azure moonstone gives +6 spirit. Both are blue gems, which preferably should be put in a blue socket, if you want to get an additional bonus. The peridots are green, which in this case means you can put them either in a blue or in a yellow socket, and get the bonus in both cases.
In the beta there is a NPC vendor named Ushtug the Temporary standing next to the grand master jewelcrafter, selling exactly these cut gems, like the solid azure moonstone. As somebody here already remarked, the name suggests he won't be there in the live version. This leaves the market for cut gems to the jewelcrafters. Cutting gems is very easy, getting the cuttable gems is a bit of a chore, but no problem. The limiting factor will be that it takes 300 jewelcrafting to do so. The materials I blew to get to this point had a market value of over 500 gold. And with everybody wanting to be a jewelcrafter, the market value of the same trade goods will easily be twice that when the expansion goes live. Guilds will certainly want to have a jewelcrafter or two, but this isn't exactly the best profession for the casual player. The jewelry you can make up to 300 is nice, but will probably not earn you any money, will all the other people skilling up and putting up the same rings and necklaces for sale as you do. How well and for how much the cut gems will sell, nobody knows yet.