The Source
The rare metal of the deep dwarven holds — named, and few: the only metal that takes a spell without breaking it, and the one the runecasters want.
The Yield
A mine's life gives a handful; a handful gives a blade's worth; a blade's worth gives a story.
In the Workshop
The mithril is the floor of the masterwork: the metal that holds the promise is the one the story has to mine, and the mining is the one the hold has to give a lifetime for.
Cost band: master (the floor of the named) — on the ladder in The Maker's Hand, the material is the floor, and the floor is the one the mine or the marsh or the tree has to give. Recipes that take it: runecraft, the masterwork making