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If you're talking about diabosite you need to explore the area in the basement of the bar until you find a mine and then you talk to blake about diabosite then you can get a pickaxe and mine it a few times per day 

If you mean the diabosite mine under Bareshade, you need to start by talking to Harold the tavern owner. He'll tell you about strange noises in his basement and have you investigate. You'll find a hole that leads to the underground, and after exploring it a bit you'll find the ores.