Steam, Bronze, and Fire
An ultra light and strong metal, made by smelting steal with Gravite ore in a special process. Dwarves were the first to discover this process, and have kept their smelting process a secret, though there are a few Humans and Warforged who have learned how to make it as well. These men are well renowned craftsmen, as their knowledge for creating this material is invaluable for aircraft design, as well as many other fields.
(Mithril properties are just as they describe in the materials section of the SRD / players handbook.)