A filling is a substance used to fill a cavity (hole) in your tooth caused by decay. There are three kinds of fillings: gold, amalgam (silver) and composite resins (tooth-colored fillings). A gold filling is usually the most durable kind of filling. Amalgam fillings are also incredibly hardy, and they don't cost as much as gold fillings. A composite resin is the most popular kind of filling because it blends in with the color of your teeth. Tooth-colored fillings, though not as tough as gold and amalgam, look the most natural.