Updated for patch 4.3.
There are two types of Adamantite Deposits you’ll find, Adamantite Deposit and Rich Adamantite Deposit. You will need to have your mining skill at 325 to mine regular Adamantite Deposit and 350 to mine Rich Adamantite Deposit.
If you are trying to use mining as a way to make gold take a look at this Gold Course which teaches you strategies to maximize the gold you make when you sell things on the Auction House.
The two zones I’ve chosen for Adamantite are Nagrand and Netherstorm. Both of these are best done with flying mounts.
Adamantite Ore in Nagrand
The main focus of this zone is the caves which I’ve marked with V’s on the map. I would spend most of my time travelling between them and then taking the north route to start over again. If you wanted to you could also fly the southern edge of the zone to get back to the start.
Here are the two different database sources for nodes where Adamantite was recorded by people mining. (Click to Enlarge)
Adamantite Ore in Netherstorm
Netherstorm is comprised of a bunch of islands so you’re definitely going to want to have a f lying mount to mine in Netherstorm. There’s one fairly large cave you can go in marked on the map. If you set yourself up in the right position for the small northwest island you can afk farm adamantite ore. You just come back and check your computer every so many minutes and mine the nodes when they spawn.
Here is a database source for nodes where Adamantite was recorded by people mining. (Click to Enlarge)
If you’re looking for information on Leveling your Mining see my WoW Mining Guide.