In this post, I thought I would talk a little about the history of treatments and the enhancements which may have been made to gemstones set in antique jewellery.
The majority of gemstones sold today have been treated in some way to improve or enhance their appearance. The purpose of the treatment may be to clarify the stone to remove impurities, to reduce the number of inclusions, or to improve or change the colour. Other treatments may be to reduce or fill surface fractures or internal fractures. The treatments undertaken can include heating, irradiation, dyeing, waxing, glass filling, oiling, coating or a combination of these. Some of the treatments are stable and irreversible; others can lead to fading, chipping, leaking of fracture fillers and, in some cases of irradiation, to serious contamination of individuals.
Treating gemstones to affect their appearance in some way is not a recent phenomenon. The 2000 year old Papyrus Graecus Holmiensis discusses cleaning pearls by feeding them to chickens, although modern tests of the process have found that while the chicken’s gizzard was effective in cleaning the pearl, the chicken needed to be killed to recover the pearl before it dissolved in stomach acids.
The Egyptian Stockholm Papyrus, dated around 400AD, dealt with methods of improving the colour of stones and gave instructions for improving pearls, particularly pearl whitening, and ways to imitate pearls. They also covered dyeing stones, usually rock crystal and quartz, as well as heating and dyeing other stones.
Pliny in his Historia Naturalis, around 50AD, discussed foiling, oiling, dyeing and composite stones, as well as cooking stones in vinegar to make them shiny. 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 explains how pearls could be peeled to find better surfaces. It had been thought also that Pliny had talked about oiling of emeralds but now it is thought that he was referring to the oiling of turquoise. Oiling of emeralds was not written about until the 14th century.
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 citron sapphire and beryl 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, the heat treatment of sapphires to turn them colourless and the use of black backing or coating of diamonds. In Italy at this time, the use of foils was permitted but tinting coloured gemstones (other than diamonds) was prohibited. Doublets, sandwiching glass and a slice of original gemstone together, were common but not permitted. The Textbook of Mineralogy by Georgius Agricola in 1556 also discussed doublets and triplets as well the dyeing of light coloured sapphires.
At this time, the use of furnaces was common in metallurgy and would have been used to heat gemstones. In 1609, Gemmarum et Lapidum Historia was produced by Boetius de Boot from Bruges. He discussed decolourising by heat sapphires, topaz, amethyst, etc, to produce diamond imitations, the dyeing of stones, mostly with metal compounds; metal foil use and hardening of gemstones.
In 1658, Natural Magic was published in London by John Baptista Porta. It discusses the making of coloured glass imitations and various treatments such as dyeing sapphires, amethysts, topaz, chrysolite and emeralds, to turn one stone into a more valuable one.
Robert Boule published An Essay about the Origins and Virtue of Gems in 1672. It talks about heating stones and dyeing them. In 1760, Don Bernado Monton published Secretos. It contains instructions on how to soften ivory and to make it look like new, to bleach diamonds, to harden stones, to whiten pearls, alabaster and marble, to colour marble, and so on.
The 19th century saw the publication of proper gemmology textbooks. Again, they talked about heating, clarification of amber, dyeing of agates, use of foils, etc. Max Bauer’s Edelsteinkunde, published in 1896 and translated into English in 1904, remains a valuable text. It discusses heat treatment, dyeing, creation of doublets, use of foils, oiling of opals and treatment of them to become black, bleaching, resin impregnation, amber clarification through heating and oiling, pressing of amber to create ambroid; peeling of pearls to recover a lustre. By this period, the foiling of gems is becoming less important as faceting becomes more widespread.
X-rays were discovered in 1895, radioactivity in uranium in 1896 and gamma rays in 1990. Within ten years, there were studies published on the effect of these rays on gemstones. By 1927, it was known that irradiation could produce yellow, green or blue diamonds; yellow in corundum; red or yellow in tourmaline; orange in topaz, and green in kunzite.
In 1926, Michel discusses doublets, heat treatments (brown topaz to pink, zircon to colourless or blue, amethyst to yellow); dying of agate, chalcedony, opal and the like as well as the production of dendritic patterns by the use of silver nitrate solution. He also discussed the bleaching and dyeing of pearls.
Recent irradiation treatments have seen the perfection of the process of production of fancy diamonds and irradiation of beryl to produce maxixe types. There is also laser drilling and engraving; a process to produce lavender in jade; bleaching of black coral; heat treatment of corundum from low quality roughs; diffusion coating to add colour and/or asterisms, and finally, glass filling, particularly of rubies and diamonds.
So gems in antique jewellery, if treated, might have been heated to clarify them or to enhance the colour, or might have been dyed, again the affect the colour. Emeralds would have placed in oil just after mining. Over the next few blogs, I will talk about paste gems, imitation gems and synthetic gems.
Reference: K Nassau, Gemstone Enhancement. London and Boston (Butterworths), 1984.