Celestium is a very rare metal that is likely highly ferrous, considering its properties exhibited when in contact with nether. It is rumored to fall from the sky on occasion.
Celestium does very high alchemical damage to nethrim, and greatly resists the effects of nether. Weapons and armor made from it are highly prized for these reasons, particularly amongst the Knights Templar.
Note: Celestium was originally called meteoric iron.