Now is a really great time to be a jewel crafter, at least on my server. Obsidium will often reach prices of around 80g per stack, and rarely go much above 100g per stack. And uncut green gems don't usually fall below 15g, with the more popular ones (red, orange, and purple) staying within the 20-50g range. The best thing though, is that they're still selling like crazy. I'll post around 20 of each (except carnelian and sometime jasper, more on that in a second) and rarely will I get them back and only occasionally will I be so undercut that I have to cancel and repost.
Here is some quick math I hope will help illustrate why simply prospecting and reselling is so profitable. At the average best I see Obsidium for 80g per stack, and given an average of 6 green gems per stack, that means each green gem costs me 13g33s to "make." Even if the lowest I can sell them for is 15g, that's still straight profit (though not much). But I can usually sell at least the orange and the purple greens for three times that at the least. And this doesn't even take into account the occasional rare gem you get, nor the many other things you can use these gems for to make yourself even more money!
Right now, the best use of the gems I get from prospecting is to make Shadowspirit Diamonds on my alchemist. Three of each gem equals two of the Diamonds. Yes, each transmute is a guaranteed two Diamonds, even if you're not a transmute specialist. Given the base "cost" of what it costs me to make the green gems from prospecting, it only costs me a few copper under 120g to make one Diamond (3*6 green gems * 13.33g per gem / 2 Diamonds per transmute). And this isn't taking into account the procs I get from being a transmute specialist. The raw Diamonds on my server sell for around 300g each, but take it one step further and cut them! My best selling cut is the Ember Shadowspirit Diamond (around 400g) and the most expensive I can make is the Revitalizing Shadowspirit Diamond (around 500g).
The other thing you can usually do with gems is to make items to disenchant. Right now, Jasper Rings and Carnelian Spikes are the best to make and disenchant. One jasper and one setting make a ring that can be disenchanted for what seems to be an average of 2 Hypnotic Dusts and occasionally Lesser Celestial Essences. I should do some more research to get the exact numbers, but I usually won't bother making and disenchanting unless the cost to do so is less than the cost of one dust. These are also a somewhat decent way of getting Heavenly Shards. Occasionally you'll proc a blue ring, which will DE into one Small Heavenly Shard (3 make a normal Heavenly Shard). The spikes take three carnelians and three settings, but give an average of two Greater Celestial Essences per disenchant, with the occasional unlucky 4-5 dusts or the lucky 3 GCE. These are a great way to get GCE and are usually quite profitable.
For a handy way to calculate your expected profit from prospecting ore, here is a wonderful spreadsheet I picked up over at the Consortium Forums:
All you have to do is change the price of ore and green and blue gems to match your server (and however much a Shadowspirit Diamond is selling over there), and bam! you have your expected profit.
Credit for the spreadsheet goes to drockrock, using information provided by Stede over at Venture, LLT, with a few modifications by yours truly to include the cost of making your Fire Prism (I recommend not doing this for a while), and the cost of making a Shadowspirit Diamond and your expected profit selling it raw. I'd like to add more to it eventually, and I'll post an updated one if I ever do so.