Farming for Volatile Earth in Twilight Highlands
The Obsidian Stoneslaves in Twilight Highlands are a great source for Volatile Earth. You can find them in The Black Breach, just northeast of Bloodgulch and east of Thundermar. Around the center of The Black Breach you will see "sundered ground" with purple light coming out of the center of it. The most commong place to find the Obsidian Stoneslaves is on the land immediately surrounding this crevace. However, you can also find them patrolling the outter area of The Black Breach but they are escorted by Twilight Cultists, meaning you'll have to kill an addionational mob with no chance of dropping Volatile Earth.
Also an added bonus if you're a Miner is you can mine the Obsidian Stoneslaves here. When you mine them you have a chance to get some extra vendor trash as well as a chance at even more Volatile Earth.
Volatile Earth Farming Video: