Foil
a leaf or thin sheet of metal placed beneath transparent jewels to heighten their color and improve their brilliancy; also applied to those sheets of tin amalgam placed behind mirrors. They are made of copper, tin, and silvered copper. The sheet lead which is used for the lining of tea-chests is a species of foil. By extension, anything of another color, or of different qualities, which serves to adorn or set off a thing to advantage.