Now, there are countless guides that are: "Buy the tabard or potions over and over and then resell them for gold," but this one is not related to the tabard or potions. What this requires is an enchanter who is revered with Ogri'la and who has plenty of shards, if you're guilded you should be able to find one easily.
If you're level 70, then chances are you do the Ogri'la and the Skettis dailies regularly, personally I do all of them every day, since they're much easier than the Netherwing, and quicker to take care of, not only that but until a couple weeks back when I started to do the trick about to be described, they'd get me lots of free Healing Potions, however, there's a much, much better use to the Apexis Shards than that.
Requirements:
You need to have 35 Apexis Shards, minimum.
Your disenchanter needs to have 50 Apexis Shards, minimum.
You need to be able to two/three man an elite mob effectively, either by burning it quick or healing a tank.
How does it work?
If you ever completed the "A Fel Whip For Gahk" quest, then you know that the Apexis Crystal Prisms located at each of the Forge Camps in Ogri'la summon an elite demon who drops the quest item, as well as an Apexis Crystal. To summon these mobs it requires 35 Apexis Shards, and when you get an Apexis Crystal, you can then purchase a BOP blue if you're revered with Ogri'la, which can then be disenchanted into a Large Prismatic. Almost 100% of the time the mobs will also drop a BOE green. On top of the 20 prismatics we earned three stacks of arcane dust, and 7 greater planar essences, as well as an extra Large Prismatic due to luck.
Important Info
It costs you 35 Apexis Shards each time to summon one of the demons, however to buy the blues you need another 50 Apexis Shards, so your enchanter needs at least 50 Apexis Shards for however many crystals you gather.
Apexis Crystals are BOP and no matter what you need to pass them to your disenchanter.
What to do
In order for this to work, you need to be able to kill the elite mob without much difficulty, otherwise you'll just wind up wasting shards, time, and most of all, money. I was able to two man this by logging on my Shaman alt on a second account, and tanking it on my warrior while my rogue friend (who was an enchanter) DPSed the mob and then picked up the crystals. I was Fury specced in prot gear dual wielding to hold threat, and I was healing myself by tabbing in and out, so I doubt that an actual protection warrior/feral druid or any tanking alternative + healer would have much difficulty. If you have a third man with plenty of Apexis Shards, then it would be worth it to get him to come, but we got 20 Large Prismatic Shards in just over one hour, using the method that I described above, and split them down the middle. On my server that's the equivalent of 500g an hour, and although it was divided, it's great to earn a Mongoose enchant out of the thought to be worthless Apexis Shards.
Now, when it comes to actually killing them, head north to 31,41 at the Forge Camp and you'll find an Apexis Crystal Prism, this is what is required to summon the mobs, a little background info on each of them:
Braxxus - Easily the hardest of all the mobs, he has an AOE fear. Tremor totem or fear ward on the tank would be great. He's also a bitch to position and has an Ony-type hit box, where you can stand 20 yards back and still hit him. Interrupt his Double Breath and he should drop without problem.
Mo'arg Incinerator - The easiest of all the mobs, his only real ability is his Acid Geyser, which is a cone ability that does damage to all targets in front of him, interrupt it and he's no problem.
Zarcsin - Buffed up version of the doomguards at the southern camp. He does a fire cone in front of him which is interruptable, he enrages at 50% but the damage increase is minimal.
Galvanoth - Big version of Magtheridon, has a cleave and an AOE knockback, if you're at the north camp don't face towards the cliffs, interrupt the fel flamestrike, and time your heals when the MS is wearing off or off the tank altogether. He hits the hardest but isn't too much of a problem.
Dropping any of those mobs will yield an Apexis Crystal, which is basically a Large Prismatic. 35 for the summon and 50 at the vendor = 85 shards per Large Pris, giving these useless things an easy use. Myself and my rogue friend do this at the end of each week. We average around 750 shards between the two of us which is a quick and easy 8 prismatics.