Foil
foil | ˈfȯi(-ə)l
noun
The reflective coating added tp the back of a gemstone or rhinestones to increase brilliance and depth of color
Foiling of a gemstone involves applying a thin sheet of metal or other similar material (occasionally of a different color) to the back of a gemstone in a setting to reflect more color into the stone. The result is a brighter-looking stone in the piece. A foil can be a reflector only or it can be used to color the stone differently from its natural coloring. The result is that foiled gems are most often set in closed backs to protect the foiling from deteriorating over time and to hide it from those who might see the augmentation. The first known example of foiling dates back to 2000 BC but it became popular in the 18th and 19th century. Today, it is rarely used except in the case of rhinestones.