Mithril - No one knows exactly when the Dwarves discovered Mithril in the mines of Moria, but it was likely sometime during the Second Age after the Naugrim of Beleriand migrated to Khazad-dȗm, adding to their numbers, and before the reign of Númenórean king, Tar-Telemmaitë, who eagerly sought to accumulate as much of it as he could.

"'For Mithril,' answered Gandalf.  'The wealth of Moria was not gold and jewels, the toys of the Dwarves; nor in iron, their servant.  Such things they found here, especially iron; but they did not need to delve for them: all things that they desired they could obtain in traffic.  For here alone in the world was found Moria-silver, or true-silver as some have called it: mithril is the Elvish name...Mithril!  All folk desired it.  It could be beaten like copper, and polished like glass; and the Dwarves could make a metal, light and yet harder than tempered steel.  Its beauty was like that of common silver, but the beauty of mithril did not tarnish or grow dim.  The Elves dearly loved it, and among its many uses they made of it ithildin, starmoon, which you saw upon the doors.'" ~The Fellowship of the Ring, Book II, chap.4 (A Journey in the Dark)