It looks like there's another wave of cheap duped goods starting up now. There have been reports from several servers about being flooded by level 1 toons selling stacks and stacks of Queen's Garnet very cheaply. If it happens on your server don't be the sucker who buys the first few cheap stacks because it won't be long before they're selling even more for far less
There have also been a few mentions of cheap Essence of Destruction showing up in suspicious numbers on a few servers.
You've probably noticed some colors of epic gems sell better than the rare equivalents and others worse. Queen's Garnet (red) sell like crazy, Shadow Spinel (purple) hardly sell at all and other colors sell better than purple but nowhere near as well as red.
It's the same for specific cuts, some cuts that sell well as rare cuts hardly sell at all as epics. Everyone wants Queen's Garnet (red), the other gems don't sell nearly as well. Shadow Spinels (purple) have been hit the worst. Cuts like Veiled, Glinting and Etched hardly sell at all and prices are dropping fast. Yet those same cuts rare versions still sell quite well. It's the same with other colors and cuts too, Rigid Deepholm Iolite for example hardly sells at all, Sparkling, Solid and probably even Stormy far outsell Rigid... Yet Rigid Ocean Sapphire still sells well.
Ever wondered why?
The answer is in the itemization of new ilevel 397+ gear in patch 4.3. In particular PvE DPS gear has mostly red sockets, a few yellow sockets and absolutely no (zero) blue sockets. Tank and healer gear do have all 3 sockets colors though healer gear is still mostly red sockets.
Consequently the vast majority of players (DPS) no longer have any use for blue gems or hybrid purple gems (fits red and blue sockets) in the new gear. For yellow sockets they'll use an orange Lava Coral if the socket bonus is worth getting, otherwise they're just going to gem straight red for their primary stat.
Healers and tanks still use a few blue and blue hybrid cuts like sparkling and puissant but the old rare best sellers like glinting, veiled, etched are DPS gems. Even purified doesn't sell very well because it's only healers that might use them rather than all casters. Tanks will also use a lot of Fractured Lightstone but most DPS and Healers are probably going to use an orange Hybrid rather than a yellow gem. For healers and dps 25 of your primary (red) stat plus 25 of a secondary stat is better than 50 of a secondary stat.
In a nutshell, the vast majority of gems are purchased by dpsers. Dpsers now buy mostly red queen's garnet and the occasional orange lava coral. Demand for green, purple, blue and yellow is primarily from tanks and healers. When it comes to healers they buy more red than anything else meaning the demand for non red epic gems is mostly driven by tanks... The role with the least representation in raids.
Blizzard screwed up an already screwed up market even more. They listened to DPS QQ about wanting more red sockets, no blue sockets and gave in, thereby further increasing already huge demand for red gems, decreasing demand for all other colors and effectively made purple gems just another cheap garbage gem. Prices of course follow demand, shadow spinel have the least demand and lowest prices, queen's garnet have the most demand and highest prices.
So there you have it, now you know why epic gem prices are the way they are and why the best selling epic gem cuts aren't the same cuts as the best selling rare cuts.