Foiling of gems was a very common practice for centuries, with examples of foiled stones being found in Minoan graves around 2000AD. The use of foils to deepen the colour of stones was accepted at various times in history. Foiling continues to be used today but really only for crystals in costume jewellery. Placing coloured metal foil at the back of gemstones which were in closed settings was a common practice during the 17th and 18th centuries. In particular, foiling was used to assist jewellers produce matched gemstones as the foil could intensify colours.
While the main objective of foiling was to increase the colour of the stone and to increase its brilliance, it was also used to impersonate another gemstone. In the 17th century, diamonds, emeralds, rubies and sapphires were used in both rivieres and in esclavage necklaces. For those who could not afford these gemstones, rock crystals which had had the backs covered in foil could create the effect of these precious gems. Foiling of rivere necklaces in particular continued well into the 19th century.
The foil used during the Georgian period was a thin sliver of copper tinted to the colour of the gemstone being used or impersonated or silver in the case of diamonds. Sometimes these foils could tarnish or fade. In the 19th century, foil made from a thin leaf of tin which was then coloured was used. Aluminium was patented in 1889 with aluminium foil becoming commercially available in 1910.
Foiling could either involve lining the back of the setting or placing the foil on the back of the stone. The foil might be a silver colour to help reflect light through the stone or a colour the same as the gem to enhance it. The process used could be as simple as just a small square of foil being placed inside the closed setting, an act of a few seconds. Open settings became more common during the Victorian period and so foiling of gemstones became more uncommon. One gemstone that continued to be commonly foiled was the garnet. Cabochons and rose cut garnets used in Bohemian jewellery, for instance, continued to be foiled to enhance the red colour of the garnet.