Have you ever heard someone say - "I don't want to gem it or enchant it - I will replace it soon!!" ?
Many players want to be 'the best' or will try to tell you it doesn't matter. Most toons - seeking red slot gems; use inferno rubies or nothing.
(new cut stats taken from db.mmo-champion.com)
A cut JC chimera eye will give + 77 stat
A cut JC Chimera eye currently gives +67 stat.
A cut Queens garnet will give you +50 stat : costing???
A cut inferno ruby gives you +40 stat : costing 100-140g
A cut carnelian gives you +30 stat : costing 30g.
Even a cut hessonite gives you +15 stat plus something usefull : costing 8g; half a quest reward.
Now, epic gems in Cata won't be like Wrath; being gated they will be very expensive at first while toons are replacing gear.
Bleeding edge raiders may drop a profession and take up JC'ing now; and will get (I think) +30 stat on day one.
I will be gemming everything I get with what I think is worth
gold/point. I expect that is likely to be inferno rubies for the first
month of 4.3. My (non raiding) JC will love the extra bonus but doesn't have the gear to really benefit from it
Joe average will get a lot of gear from the new raid finder. However, I expect John Pug to decide they can't afford the best so will leave the slot empty. Because of this, I expect the gating to be unlocked 1/2 way between 4.3 and 5.0. (WAG : I'll predict in March 2012 you can prospect Epic gems from Pyrite Ore.)
There are toons that won't wear blue stuff. I do expect to sell more Northrend epic gems. 'Cheap Epicnezz lol' to keep you purple.