Foilbacks is a vintage jewellery term that refers to the practice of inserting metal foil behind gemstones or faux gemstones to enhance their sparkle and reflective properties. When this foil darkens or peels, these gemstones are often considered dead or lacking in sparkle. Modern jewelers seldom use foiling behind actual gemstones, but faux gems are made in a similar fashion even today.
References
Foilbacks Wikipedia(Text) CC BY-SA