Base metal is any metal that is not one of the precious metals such as 925 silver, 14kt gold and white gold. That is what I would consider fine jewelry metals. For example, if you purchased something from 'Cartier' or 'Tiffany's' more than likely it would be made in precious metals (the good stuff.) In costume jewelry base metals are often plated with a very thin layer of gold, silver, nickel, rhodium, or other metal on the surface of the bead, ear wire, findings or other components used in costume jewelry making. Although, base metals can often be misleading it's wise to know for certain what materials are being used.