Sculpted by: Israel Skelton
Release: 2011
Edition: Limited
Used material(s): Cast in dull black nickel-plated pawter
One of: 1000

Signed: Israel Skelton

First appearance: Clockworks #3
The Whispering Iron is the material that the Keys are made of.

More information:
(possible spoilers)

The Whispering Iron was first discovered by Benjamin Locke in 1775, during the Revolutionary War. A small army hiding in the Drowning Cave had an encounter with the Black Door, during which Harm Timmerman was possessed by a demon. Other demons were unable to find hosts, however, and fell dead, turning into Whispering Iron.

Benjamin used the first piece of the Iron to forge the Omega Key to seal the gateway shut, stopping any more demons from entering the world.

Source: Locke & Key Wikia