Thorium Bar

Now that TBC has been released and settled into the WoW world, this stuff has skyrocketed in price. More people seem to be taking dual crafting professions and it is used in Engineering and Blacksmithing, the two popular ones for getting a big mainhand weapon and an epic head item. Less people are mining this as it drops in the old world and the demand has risen on Earthen Ring EU while the amount farmed is going down. I was suprised to see a stack of Fel Iron Bars going for 6-7g less then Thorium Bars today.

This stuff seems to be a money maker with the large number of people rerolling trade professions to get epic goodies. Only question is how long will it last until things settle down after 2.1.0 revamps and the price stabilises?

EDIT: This guide is less accurate now due to the changes of thorium spawning in Un'Goro. Walking around in the caves of Silithus is much more time-consuming then using ur 100% mount in Un'Goro. However, if Un'Goro is crowded by farmers, Silithus is a good second alternative.
Thank you 'spiralingshape' for your info

Thorium, the radioactive metal Engineerers and Blacksmithers would pay a fortune to get their hands on.
Since most Tanks have blacksmithing as profession, which needs a lot of thorium for skills, makes this is an extraordinary item to farm.

At level 80 you could without problem mine 10 stacks of these/hour (+/-). You only need to know where to farm them.
I'm not a big fan of bugs & insects, but my experience tells me that Silithus is by the far best place to farm thorium ore, becuase of the fast spawn rate and the numbers of Rich veins. There are three areas you wanna visit in Silithus to quickly earn thorium ores.

Hive'Ashi - In the North
Hive'Zora - In the West
Hive'Regal - In the South-East

The thorium does not often spawn on the surface in these 3 areas, you'll have to go underground to find the real treasure.
Each area have 3 caves with big tunnel systems, in each cave you will find 1-2 Ooze Covered Rich Thorium Vein which will give you average 5xThorium Ore. So, with simple math you should get 30x Thorium Ores by only visiting all three areas + the ores you may find travelling between the areas. Of course, if there are more players farming them, you'll get less.

After mining from all areas, the ores in the first area you visited have respawned. This makes Silithus a non-stop thorium farming place, no matter if you are level 60 or level 80.
While farming, you will not only make money from the ores, but also from the rocks and gems that may drop.
Dense Stone
Arcane Crystal
Star Ruby
Azerothian Diamond
Blue Sapphire
Huge Emerald
Large Opal

Also, to make some extra money suggest getting Agent of Nozdormu or ask a friend who has it give you Proxy of Nozdormu, these items will make you able to loot these from the Hive-mobs which can be turned in for reputation with Brood of Nozdormu or sold to other players.
IF you don't have the item I recommend you to NOT waste your time looting any Hive-mobs, since they drop no kind of cash nor item worth selling. However, there's a low chance that they will drop Brann Bronzebeard's Lost Letter which gives Brann's Trusty Pick as reward.

(I would love to get some ratings & responses to confirm this "guide")

Enjoy your farming.

A good place to farm for thorium (as well as arcanite crystals) is in the Dark Wisper Gorge, in southern Winterspring.

The area is filled with elite demons but if you are level 60 getting around unharmed is very doable. Stick to the sides of the area and be ready to run for a clearing if you get spotted. As a warrior, i put on my tanking gear and whatever mount speed increases i could find and was able to get around to all the ore there with a little practice.

Go through the tunnels carefuly because demons walk through them periodically. Also try to memorize the layout of the area, it can get a bit confusing. Do what you can to not die (get potions if you need to) because the run back is very long.

Oh, and make a stop by the owl wing thicket every time you pass it. A large Thorium deposit spawns there in one of the caves.