Tantalum is a rare, blue-grey metal that is found deep within the chasms of Ferchiun*. Immune to all but the strongest acids, if more had heard of it would be more sought after. It is nearly as hard as adamantine and holds an edge equally as well. Those few who believed such a tale did not return from their quest to gather any, or if they did, it was not to the homes from which they came.
Medieval Britons didn't write contracts. Instead, men making agreements would clap their knives onto an altar and recite the agreement three times to seal a deal. Even after the Normans introduced written contracts, British nobles would wrap the parchment around a knife to authenticate it.