1.) Copper, Nickel, Manganese, and Aluminum are not a semiprecious metals and are commonly used in gold alloys.2.) You probably just have not had any experience dealing with this stuff. It may seem counterintuitive for you, but it's always done by percent mass (since the days of Archimedes) because mass is explicit. 18K gold is always 3/4 gold by mass. If you know the mass, you know the fine weight, and therefore the minimum value in gold.