A technique of foiling by which a colourless transparent stone was painted on the back with an amalgam (an alloy of mercury) in the manner of a mirror, so that the amalgam was in contact with thestone, unlike foiling with a metallic leaf that was attached to the supporting metal. It has been used since c. 1840, especially to give a colour to paste and so that it could be set in an opensetting.
Mirror Foiling

Mirror Foiling
A technique of foiling by which a colourless transparent stone was painted on the back with an amalgam (an alloy of mercury) in the manner of a mirror, so that the amalgam was in contact with thestone, unlike foiling with a metallic leaf that was attached to the supporting metal. It has been used since c. 1840, especially to give a colour to paste and so that it could be set in an opensetting.
References
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson



