The Precious Ores is an event in Act I of Diablo III. It is given by Tashun the Miner, found in level 2 of the Lost Mine.
The Lost Mine is a random dungeon found in the center of the Fields of Misery. Tashun assigns the player(s) to find a Vein of Metals and a Pure Gemstone. The most common monsters are Plague Carriers.
Once the quest is complete, Tashun will be willing to buy, sell, and repair items.