Paillon, French for "foil". A thin sheet of metal (gold, silver, copper or an alloy made by various formulae depending on the colour desired to match the stone to be foiled), used in jewelry as a backing to certain mounted gemstones to enhance the color of brilliance.
Foil has been used from earliest civilizations. 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. In the 16th century it was sometimes used to back a diamond (cut as a point diamond or a table cut diamond) so as to add to its brilliance, but usually a diamond is not foiled but occasionally tinted.
Foil has also been used to back garnets in Anglo-Saxon jewelry and in jewelry of the 17th to 19th centuries. Sometimes, to heighten the brilliance, the foil is pricked or crosshatched.
When beaten very thin, foil is known as leaf, and is used as a decorative covering for cheaper metals. Sometimes foil has been used deceptively to give to some inferior material the appearance of a gemstone.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson