Can Bone of the Ancients boost any element?

I've been farming loot midgets and all of the Bone of the Ancient Relics have been + Shock Damage. Is it always Shock Damage or can it be other elements as well?

Edit: Okay, I got a Corrosive and Fire now. Took me a few runs, but its worth it. I'm also pretty sure that Slag is not one of the possible elements.