Onyx Lodestone Farming Guide
There are two locations you can get Onyx Lodestones to drop, one of them is in Malchor's Leap and the other is in Cursed Shore. Only one of these locations is reliable and that's the one we're going to be taking a look at in Cursed Shore. West of Pagga's Waypoint you'll find a large open area with a stone pillar surrounded by Cypress Saplings (which contain Elder Wood Logs when gathered). This location here has a chance to spawn a few different mobs. Commonly those mobs here are Earth Elementals and Spirits.
As you could have probably guessed the Earth Elementals that spawn here are what we are after. However they won't always be here, as aforementioned and as I go over in my Onyx Lodestone Farming Video. From what I have seen it depends on when your server comes back up after a crash, restart or patch. Really anything that brings down this specific zone the mobs have a chance to change here. Aside from that as far as I know there is not a way to change what spawns at this location.
Below is a picture of where you can find this location on the map of Malchor's Leap. You may need to click to enlarge it so you can view it better. Although it's only about 100 pixels smaller in width. (that's what she said!!)
You may not get an Onyx Lodestone every hour from farming here but you'll be getting a lot of other goodies such as White items you can salvage or vendor, Gray items that you'll be able to vendor as well as Greens and Blues which can both be vendored too. More noteably though are the Emerald Shard and Emerald Crystals you'll be getting from here. These sell on the Auction House for a little bit more than their vendor value and add a nice cushion, I like to call it, if you don't find anything good.
You find on average 1 Emerald Crystal every 5 or so kills which is 1s 10c every 5 kills. At the bare minimum. Even if you don't find any Onyx Lodestones here combine the Emerald Crystals/Shards and all other vendor trash together in an hour of farming and you still have around a 1g profit.