Hi there, foil that is applied behind pastes is made of a thin sheet of material, the actual material depends on the color desired to match the stone to be foiled. So to answer your question, the material used for foils could be gold, silver, copper or an alloy made by various formulae.
Foils used in jewelry as a backing to mounted gemstones are used to enhance the color or brilliance of the stone.
Foils have been used since ancient days. It is made by hammering sheet metal to the thinness of paper and is applied by beating it to a metal object or by means of a fixative.
When beaten extra thin, foil is known as "LEAF" and is used as a decorative covering for cheaper metals. The French term for foil is "PAILLON".