Synthetic and imitation opals
The first attempt at manufacturing synthetic opal was made soon after the determination of the structure of opal in 1964 Sanders. Details of the procedure used to first synthesise opal are preserved in the original patent documents registered by the CSIRO in Australia, Great Britain and the USA during 1964. The original patents for the synthesis procedure were finally accepted some seven years later, on October 11th 1971.
During this time Pierre Gilson Laboratories were also involved in research into the synthesis of opal, and Gilson’s first opal was manufactured in the late 1960’s; but this man-made opal was of very poor quality. The first commercial production of ‘synthetic’ opal was not manufactured until 1975, with the introduction of the Gilsonô created gemstones, ‘synthetic’ opal was a reality and it began to appear in items of jewellery with its properties and features being reported in gemmological literature.
Hearsay suggests that all of the earliest attempts at manufacturing synthetic opal were dogged by instability of the final product that resulted in the opaline material cracking and crazing.
Today, the accepted procedure for manufacturing both synthetic and imitation silica based opals is essentially the same as those recorded in the original CSIRO patents. Essentially, this consists of:
A method of producing suitably sized spheres of silica.
A method of settling and separating randomly sized spheres into uniform sizes, and depositing or precipitating these into an ordered array.
A method of solidifying, aggregating, dehydrating and compacting the array into a stable product.
Man-made simulants of gemstones may be separated into two groups: synthetic gemstones that are an exact chemical equivalent of the natural material; and imitations that are simply look-alikes and otherwise chemically distinct from natural opal. Imitations can be further subdivided into those imitations that have a very similar chemical composition to the natural material, and those that have an entirely different chemical composition but have a similar appearance to the material they are meant to imitate.
Gilson Synthetic Opal
Early production of synthetic opal from the Gilson Laboratories from the 1970’s was a material that had a distinct pinkish background to its play-of-colour. However, this manufacturer soon refined his process so that he was able to produce both black, and white ‘synthetic’ opal. Specific details of the Gilson manufacturing process for opal have never been revealed. However, some specific detail about this material is available in literature. According to Darragh & Perdrix, the arrival of Gilson ‘synthetic’ opal in the marketplace was announced in 1972 by a New York jeweller who stated this opal would be available for sale in early 1973.
Later production runs of Gilson white, crystal and black ‘synthetic’ opals gradually improved the visual appearance of these opals to such an extent that while the distinctive ‘lizard skin’ appearance of this synthetic opal has never been removed, it is often cunningly disguised by cutting cabochons at an angle to their basic columnar structures. However, close observation always reveals the presence of these tell-tale structures in Gilson ‘synthetic’ opals.
In 1980 the Gilson patents and processes were sold through Dr Nakazumi to a Japanese company, Earth Chemicals. It is a fact that while the original Gilson ‘synthetic’ opal disappeared from the marketplace, it has more recently been replaced by a Japanese product based on the Gilson process of manufacture. The trend that followed has seen the appearance of man-made opal with large sized colour grains (colour patches) within the pattern of the opal, and with the availability of distinctly more presentable blue-green coloured ‘synthetic’ opal as well as the more usual red predominant variety. In both instances, while the identifying ‘lizard skin’ pattern and the columns can be observed in these opals, they are becoming distinctly more difficult to find.
In time details of a slightly different chemical nature of newer Gilson ‘synthetic’ opal were revealed in the literature. Schmetzer and Simonton et al. revealed the addition of small spheres of zirconium oxide (ZrO2) to the mix of silica spheres on which this product was based; the deliberate depletion of water from these man-made opals; and the presence of some organic material in Gilson black ‘synthetic’ opal.
This additional information provided grounds for some very interesting speculation and perhaps also offered further aids for identification to the gemmologist. For many years, there has been much discussion concerning the cause of the black colour in the natural opals from Lightning Ridge. Until recently, it was doubted if sufficient organic material could be found in black potch opal to provide it with its black colour. However, with their hypothesis that microbial action could be an etiological agent in opal formation, a new theory for opal formation proposed by Behr and Watkins suggests that the presence of organic material, as a possible cause for the black colour of opal, remains a possibility. However, Simonton et al. suggested in his analysis of black Gilson ‘synthetic’ opal that the amount of ‘carbon’ they detected was not sufficient to cause the black body colour of that opal.
Simonton et al. refer to the Gilson ‘synthetic’ opal as ‘Gilsonite’, and also reveal a structure for this ‘synthetic’ opal that consists of small crystalline spheres of zirconium oxide (zirconia) that are present in the interstices between the close packed array of silica spheres. One can only speculate that the zirconia is used either to further promote the ‘diffraction grating effect’ by a change in refractive index, or to add to the stability of the silica sphere structure.
It is interesting for the gemmologist to note that when ‘crystal’ opal varieties of these all-silica ‘synthetic’ opals are viewed in transmitted light, they show (Figure 6.) a slightly pinkish body colour. In contrast, all the natural varieties of these types of opal show a distinctly yellowish body colour (Figure 7.). This difference may also be caused by included zirconia or polymer in the man-made opals, whereas according to McOrist and Smallwood the slightly yellowish body colour in natural opals may be due to inclusion of other trace elements of which iron and aluminium are a possibility as they are often present in these opals.
Inamori and Kyocera synthetic and imitation opal
The introduction of the Kyocera synthetic and imitation opal products has stimulated less interest in gemmological literature. Schmetzer & Henn reported on this product and discussed and illustrated that these man-made opals displayed similar characteristics, compositions and structure as the Gilson ‘synthetic’. US Patent documents, registered by the Kyocera Corporation (Inamori), describe the process of manufacturing of synthetic opals in some detail. This patent includes details of how zirconia is included into the sphere structure of man-made opal. This patent also lists the purpose of this deliberate inclusion as giving the material a “markedly improved weather-ability, heat resistance and chemical resistance”.
It is interesting to note that the Kyocera patent describes a slightly differing form of the zirconia in the manufacturing process, as the zirconia is ‘precipitated’ into the voids between the silica spheres by chemical processes; where as Simonton et al’s paper suggests the deliberate inclusion of zirconia ‘spheres’. As these processes all seem to be occurring about the same time, it becomes difficult to determine which manufacturer is responsible for which material; and one has to wonder as to the competition that may have been playing a part in the production of these synthetic and imitation materials. These patent documents also reveal the reason for changing the composition of previous attempts at ‘synthetic’ opal production, from production processes using plastics or polymers. It was noted that these ‘polymer’ imitations degrade over time with the polymer component “turning yellowish” with age.
At about this time, another form of imitation opal had started to appear. This material was a combination of silica and a polymer mixture, a polymer-impregnated silica imitation opal. This imitation opal appears to be first discussed in literature in 1984 by Gunawardene & Mertens and Schmetzer. The first indications were from one sample labelled as a ‘Mexican synthetic opal’ by Gilson. This material had a “yellowish brown body colour”. This imitation opal was tested by thermogravimetric analysis (TGA), and shown to have a composition of approximately 16 % of some organic compound. More recent marketing of Kyocera and Inamori ‘imitation’ opals reveal its availability in many, often quite artificial, body colours. Figure 8 reveals that these body colours are due to the appropriately pigmented polymer components of these imitation opals.
It would seem there have been several producers of synthetic and imitation opal from Russia. Henn et al. reported on a then new Russian ‘synthetic’ opal production in 1994, as well as new Chinese imitation opal. Their paper, in German with an English abstract, gave details of these man-made opals complete with electron micrographs of the structure of these opals. Testing indicates that the Chinese material is a polymer type imitation opal with similar characteristics to the Japanese Inamori imitation opal discussed above. However the electron micrographs of the Chinese material show a distinct difference in the sphere structure.