Element Ore can be mined from small, dark rocks with a purple crystal inside. These rocks can be found on the surface and in the Grave of the Lost.
They can also be found in Crystal Isles by going into the large bee cave, then going down into he water near the back of the cave, and swimming down to a hidden cave. The nodes are the same as the ones on Aberration. You will need SCUBA Armor and/or a cryopoded water tame or a omni-tame such as Baryonyx.