Pliny issued a clear warning to purchasers of gemstones.
‘There is considerable difficulty in distinguishing genuine stones from false; the more so, as there has been discovered a method of transforming genuine stones of one kind into false stones of another… [T]here are books in existence, the authors of which I forbear to name, which give instructions how to stain crystal in such a way as to imitate smaragdus [emeralds] and other transparent stones, how to make sardonyx of sarda, and other gems in a similar manner. Indeed, there is no other kind of fraud practised, by which larger profits are made’. (quote from the Bostock translation 1898).
Gemstone enhancements and imitations were well known to the Romans. One of the first surviving manuscripts to contain recipes and methods for treating gemstones was the Egyptian Stockholm Papyrus, dated around 400AD, which dealt with methods of improving the colour of stones and gave instructions for improving pearls, particularly pearl whitening, and ways to imitate pearls. It also covered dyeing stones, usually rock crystal and quartz, as well as heating and dyeing other stones.
Pliny discussed a number of ways to enhance stones, including foiling, oiling, dyeing and boiling. He suggested boiling amber in fat to clarify it and the sugar-acid dyeing of agates and other porous stones to change their colour. He also explained how pearls could be peeled to find better surfaces. Oiling of turquoise to turn it from blue to green, the preferred colour at that time, appeared to be common practice.
In Chapter 26 of the book, Pliny noted a way to improve the colour of carbunculi (red stones) by placing coloured materials under the stone, an early version of foiling. Another method to enhance colour was to soak the stone for 14 days in vinegar.
He mentions that rock crystal could be dyed to imitate emeralds and other coloured gemstones. He warned, too, about glass imitations but commented that the glass imitations were not as cold as the real stone. He also mentioned that glass stones could contain small bubbles.
Later, he discussed the practice of producing composite stones:
‘Sardonyx, for example, is imitated by cementing together three other precious stones, in such a way that no skill can detect the fraud; a black stone being used for the purpose, a white stone, and one of a vermilion colour, each of them, in its own way, a stone of high repute’ (from the 1982 translation by Eichholz).
Pliny’s writings remained the main source of stories about gem treatments until the 14th century. In 1502, Camillus Leonardus published The Mirror of Stones in Italy, referring back to Pliny as a source. He discussed doublets and triplets; substitution of stones, particularly those set in rings, and the treatment of some stones to become diamonds. In 1540, Vannoccio Biringuccio published ‘Pirotechnia’ and in 1568, Benvenuto Cellini published Treatise on Goldsmithing’. Both books talk about the use of coloured foils and the heat treatment of stones to turn them colourless.
Kurt Nassau, ‘The Early History of Gemstone Treatments’, Gems & Gemology, Spring, 1984, 22-33
Translation of Pliny’s Natural History, Book 37 http://www.attalus.org/translate/pliny_hn37a.html