Now that we know that patch 4.3 will be the final Cataclysm content patch and contain the means with which to kill Deathwing, we know beyond reasonable doubt that epic gems will be released in that patch.
It's unreasonably doubtful that Blizzard would introduce epic gems before 4.3, and aside from not introducing epic gems at all, that's the only option that would lead to my prediction being wrong. That said, there's tons of stuff we don't know about how this will work. Here's a big one: Where will epic gems come from? If we knew that, intelligent auctioneers would start stockpiling ages before the patch in preparation.
The unique epic gems (like the Brilliant Blazejewel) are not currently available in game but were added to the client in the last patch, so we know how much better they will be than regular blue-quality gems.
Learning from history
In Wrath of the Lich King, epic gems became transmutable from blue-quality gems and prospectable from the rare ore of the day, Titanium. This was a small supply compared to the demand, so eventually epic gems became purchasable with honor. In The Burning Crusade, epic gems were rewarded mostly to guilds clearing raid content, and no effort was spent making them more easily obtainable to your average raider.
In Cataclysm, the obvious choice would be for Blizzard to make epic gems prospectable from Pyrite Ore and allow alchemists to transmute them. Of course, that's obvious enough that Blizzard might decide to come up with some spin, alternative, or variation to make us feel like we're getting a fresh, new mechanic instead of an old, stale one.
Profiting off epic gems
If Blizzard elects to make the obvious choice, the plan to profit from epic gems is clear and simple. First, stock Pyrite so you can prospect it or sell it to desperate jewelcrafters the day patch 4.3 launches. Second, stock the most expensive Volatile on your realm and Truegold, as it's fairly likely that alchemists won't be using their daily cooldown on these when they could be making expensive epic gems instead.
If you are a jewelcrafter, you should be hoarding your daily tokens so you can afford to buy cuts when they come out. If you are not a jewelcrafter, you should be buying Chimera's Eyes whenever they get posted cheaply so you can resell them when all the shortsighted jewelcrafters selling their daily token on the AH realize that they want all the epic gem cuts.
Inflation and deep pockets
The effect on the economy would be very similar to what we saw in Wrath, where at first, only the most wealthy guilds will be able to get epic gems on everyone. Considering this is going to increase their core stats by about 100, there is a point where it would be better to spend that money on BOE upgrades for most people. Eventually, the production of epic gems would start to catch up to demand and cause price drops, which would let more people use them. We may see them purchasable for honor or justice points at some point.
If I remember correctly, epic gems after a week or two in Wrath were worth about five times the old blue-quality cuts, on average. That was based on completely different market conditions than we have now, though. Our recent ridiculous levels of inflation might mean that the prices for epic gems might be horribly out of reach of all but the wealthiest for much longer. There are more wealthy people and guilds now, and the depth of their wealth is much deeper than was the case in Wrath.
There's no easy way to measure this, but if more of the wealth created since the launch of Cataclysm is currently sitting in deep pockets, then epic gems will be unavailable to the unassisted raider for a long time. If the majority of this wealth is instead sitting in the pockets of people who count daily quests and valor point bracer sales as a significant part of their income, then it won't be as long. Since an absolute metric butt-ton of this inflation was due to a probable typo in the vendor prices for uncut green gems, I'm betting on the first outcome.
Something completely different
The only reason everyone is assuming that epic gems will be prospected from Pyrite and transmuted by alchemists is that so much of the rest of Cataclysm feels like save as programming. Almost everything we see in the professions has a Wrath equivalent, so we look back to predict the future.
If the development team decides to throw us for a loop and completely redesign the epic gem system to make a better game, however, all the speculators hoarding Pyrite Ore and Truegold will be hosed. If this happens to you, whatever you do, don't sell when everyone else is panic selling. Wait until they finish, and decide whether selling before the next expansion can be profitable; otherwise, hold.
What makes a game "fun" isn't what makes it "fair," but I'm in no way qualified to be able to explain why or how. We might see Blizzard make epic gems more like Books of Glyph Mastery in Wrath or the best-in-slot bracer enchant formulas this expansion, where nobody can centrally produce them. This would make profiting on the creation of epic gems only possible by by stalking trade chat and the AH and buying low.
We may instead see something closer to the older design, but instead of epics being transmuted by one profession and prospectable from ore, giving every profession a cooldown that could create a gem. This would reduce demand, as people would simply make their own gems, and the serious profit would be in cutting them.
We could even see a design that went back to Burning Crusade logic that mostly made epic gems available to raiders, allowing raid groups to make a profit instead of auctioneers. This would still allow cutting profits but would reduce demand, as most raiders would simply get enough gems from their own group's production.
Maximize your profits with more advice from Gold Capped. Do you have questions about selling, reselling, and building your financial empire on the auction house? Fox and Basil are taking your questions at firstname.lastname@example.org and email@example.com.