Nickel

A metal found in ores that is metallurgically a poison, so its presence makes an ore deserving of more purification.

In ores

Nickel is metallurgically a "poison". Its presence in an ore makes the ore deserving of more purification.

Rumor

Some ancient holds are rumored to have made "special steels" with it. The rumored use is a direct result of dwarves attempting to recreate a material similar to star-steel.