Foilbacking is a gemstone surface enhancement technique where a thin sheet of, sometimes colored, metal is placed behind the gemstone (backing) in order to reflect more light, and/or color, back into the stone and then towards the viewer. This technique has been applied since at least Minoan times (2000-1600 BC) and is refered to as foiling.

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Georgian 9K Rose Gold Cross Pendant Set with Orange Foil Backed Quartz in Cut Down Collets.
Image courtesy of Lang Antiques.

See also