My first epic flying mount was financed to a large percentage by me mining or buying adamantite ore, prospecting it, and selling the rare (blue name) gems after cutting them for about 50 gold each. Haven't done that for a long time, so yesterday I tried it again to see where the market had evolved. Adamantite ore has remained pretty much constant in price on my server, between 20 and 30 gold a stack. But the prices for gems have changed a lot, and not all in the same direction. I sold two cut red gems for 75 gold each, while cut green gems went as low a 15 gold, and uncut green Talasites for less than 10 gold in some cases. So not only has the chance to find a rare gem from prospecting been nerfed by Blizzard, there is now also a chance that the rare gem you find is worth less than the ore you prospected it from. My try ended with me making just a small profit, and that was with being lucky with the colors I got.
The reasons for this price difference is that most people went towards a strategy of ignoring gem slot colors, because the bonus stats you get if you follow the colors is too small. So people advised my warrior for example to ignore all colors and fill up all my slots with blue +12 stamina gems. Blue and red gems are popular for this, yellow gems less so, and except for the red/blue Nightseyes all the mixed color gems are totally out of favor.
The upside of that is an opportunity for people with little cash. Mixed color gems aren't really all that bad. Two green gems for example will give you exactly the same stats as one blue and one yellow gem, but you might be able to buy them for less than half the price. And sometimes mixing stats is better than taking all the same kind. I'm not going to follow that advice to use only +12 stamina gems, because then my defense would drop even further below the magic 490 limit. So a cash-strapped warrior might be well advised to go for Enduring Talasites (+4 defense, +6 stamina) at 25 gold each instead.
I expect rare gem prices to drop in the future, especially the expensive ones. The demand is driven by the kind of people who have sufficient gold and want the very best gems possible. But with patch 2.4 the very best gems will be epic ones. The uncut gems will be sold for badges of justice, and the jewelcrafting recipes for reputation, so epic cut gems will soon be available. As for the common gems, which are already trading only at pitiful low values, apparently there will be recipes to combine them into more valuable stuff in patch 2.4. So maybe jewelcrafting can become profitable again, in spite of the usual differences of getting people to pay anything for cutting a gem. ("Looking for jewelcrafter who has a recipe that costs 1,000 gold on the AH, is willing to travel to my location, and then cuts my gem for free." -Pause- "Hey, why are you jewelcrafters so uncooperative?")