|Onyxia turns the other cheek|
Every week I take all 3 of my 90s and my 85 Blood DK out to Dustwallow Marsh to do a quick run of Onyxia's Lair (10). Actually, it takes forever on my DK, not worth the time investment for the gems alone. My main purpose is to land myself a shiny new mount Reins of the Onyxian Drake. I'm trying to avoid spending ~200k gold on a mount that I can try to farm. Onyxia doesn't really drop anything else that's noteworthy unless you're after a BOP transmog piece, an enchanter after Abyss Crystals, OR you're after the Ashen Sack of Gems. This bag of gems contains the same amount of gems on both the 10 and 25 versions of the raid. For the quickest travel time to Onyxia's Lair, get a mage to port you to Theramore if you're Alliance, or if you're Horde, hop on your mount and start flying there!! Alternatively for Alliance, you can take the portal to the Caverns of Time in Tanaris and fly from there. This will require you to port from Shrine of Seven Stars to Dalaran or use your version of the Kirin Tor rings to port from wherever you're currently standing.
Selling the Gems
You can choose to cut the gems into popular cuts or you can choose to sell them raw. However, there's more profit to be had from selling the cut gems on most servers. You can do the Jewelcrafting daily found in Dalaran to accumulate Dalaran Jewelcrafter's Token, the currency for the epic gem cuts. I have found that King's Amber and Cardinal Ruby sell the best for me. Phat Lewts of PhatLewtsGold.net has done a TON of posts on the Wrath Epic Gem Market. I highly reccommend reading through his blog posts listed below if you plan on grabbing this market by the horns. I'm just selling them because I pick them up while doing something else that I consider productive. Actually, I'm off to take 3 more toons through it!! Wish me luck :)
Phat Lewts' Posts on Wrath of the Lich King Epic Gems
JC, Mining, and Making Wrath Gold!
Long Live the Lich King!
Wrath Gems Still Profitable?