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Mining Dark Iron requires 230 skill in mining, but the only way to learn how to smelt it into bars is by speaking with  Gloom'rel, a ghost in the Summoners' Tomb in the Blackrock Depths and giving him 20  [Gold Bars], 10  [Truesilver Bars], and 2  [Star Rubies]. 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, as Hate'rel will begin the event to open the door, requiring you to kill Gloom'rel. Dead dwarves make poor teachers.

Unlike most smelting normal ore, it requires eight Dark Iron ore to create a bar of Dark Iron. Dark Iron can only be smelted at the Black Forge deep in the instance. You should take this into account, as some of the smithing recipes require as much as eighteen Dark Iron bars. That translates into at least one hundred and forty-four Dark Iron ore.

Also, Gloom'rel will remember that you paid him, so if you ever decide to temporary drop mining as a skill, you can relearn the ability to smelt it again at a later time by simply paying him a visit.

See Also: Dark Iron Bar