"Villagers gather Coin from Mines faster."—In-game description
Incan Metalworking is a technology in Age of Empires III that can be researched at a Trading Post built on an Inca settlement. Once researched, it increases the gather rate of Settlers, Coureurs des Bois, Settler Wagons, and Villagers from mines by 20%.
Incan Metalworking can be quite powerful for civilizations who send Home City Cards to stack with the technology or civilizations which already have superior mining rates (e.g. French who have Coureurs des Bois and Dutch who have faster gather rate from mines). However, this technology's usefulness is reduced if the map does not have many mines available.