When a Mine is worked, and it is not already on a resource tile, there is a small chance every turn that it will discover a new resources. A civ must have the tech unlocking a resource to gain it via a discovery. * Gems, Gold and Silver can be unlocked from the start of the game. * Copper requires Bronze Working. * Iron requires Iron Working. * Coal requires Steam Power. * Uranium requires Physics. * Aluminum requires Industrialism.