oiling is the backing of a gemstone with a metallic or non-metallic sheet to improve its optical performance. The foil acts both a reflector as well as a coloring agent. Many jewels that survived carried diamonds and gemstones that were treated in this manner. Almost always these stones were set in silver closed backs to shield the foil from deteriorating.
Of all the surface enhancements, foiling is probably the technique which has the most allure but is hardly used anymore nowadays. In present time the technique is frowned upon as we are not accustomed to any of such treatments and with an abudance of good quality diamonds and colored gemstones available at, relative, affordable prices, there is little need for a gemstone to be treated in this manner.
As mentioned earlier, the goal of foiling is to bring out the best in a gemstone. While modern knowledge can make use of mathematics to estimate the best performance, in ancient times our ancestors needed to resort to tricks and they did a wonderful job at that. Backfoiling a gemstone with a colored and reflective sheet of metal greatly increases the optical performance of the gemstone at hand.
The back foiling of gemstones is a practice that dates back to at least Minoan times (2000-1600 BC)[3] and early written accounts of it are given by Pliny[4] in the 1st century AD. In 1568 Cellini gave a technical outline on this method with recipes on how to create several foils, but the most interesting information is probably given at the end of the chapter;[5]