Foil backing (or foiling) is a gemstone surface enhancement technique where a thin sheet of metal or other material (sometimes colored) is placed behind the gemstone in order to reflect more light, and/or color, back into the stone and then towards the viewer. Therefore, the foil acts both as a reflector and a coloring agent. Most foiled gemstones are set in closed backs to shield protect the foil from deterioration (and detection?). This technique has been applied since at least Minoan times (2000-1600 BC).
A Foil Backed Colorless Quartz Cabochon.