Diatium

Diatium was a grade 4 premium scavenged metal crafting material.

Source
Diatium could be gathered from a Diatium Cargo node, which required 235 skill in scavenging. It was most commonly found on Balmorra,  Taris, Hoth, and Quesh.

The following materials could also be found in Diatium Cargo nodes:

It could also be obtained from grade 4 scavenging missions. Additionally, there was a chance to obtain Diatium from reverse engineering the appropriate level and type of gear.