User blog comment:Hallichester/The Piece of Clay/@comment-220.255.2.146-20120430140415/@comment-3481504-20120430174338

-sigh-, A listing of being who at least know about the clay.

1) God, obviously.

2) Death, obviously. Besides, he was able to describe the history and existence to the Winchesters, so I think he'd know it.

3) Archangels, as God would have informed them of it, or they'd independently figured it out.

4) Seraphs, I'd imagine that they'd know something about it, millenia of years old.