Mining Dark Iron requires 230 skill in mining, but the only way to learn how to smelt Dark Iron into bars is by speaking with Gloom'rel, a ghost in the Summoners' Tomb in the Blackrock Depths, and giving him 20 , 10 , and 2 . After you speak to Gloom'rel, a bowl will appear at his feet. Touching the bowl will give him his tribute. Speak to him again and he will teach you to smelt. It is important to talk to Gloom'rel before any other dwarf since Doom'rel will begin the event to open the door which requires you to then kill Gloom'rel. Dead dwarves make poor teachers.