Cobalt

Cobalt is a material and an element added by various mods.

Vanilla
on the Minecraft Wiki

GregTech 6
Cobalt has two variant: normal Cobalt and radioactive Cobalt-60.

Tinker's Construct
Cobalt from Tinkers' Construct is an ore which can be found only in the Nether. It spawns fairly commonly throughout all levels of the Nether in 1-4 block veins.