There is no real world material bearing the name hafaltu.
Items containing the material may cast the Multiple Ethereal Shield spell.
Hafaltu is a metal that is a dark green color and is dense and non-reflective. It is always found running through hard stones (like marble) in small veins.
It is a rare material that must be provided for alterations. Intricate detailing via carving is not possible though engraving is fine.