Photo.3a: Blue sapphire displaying blue colour, which is produced by filling of cobalt-coloured lead glass
Treatment in Blue Sapphire "Super Diffusion Tanusorn" - filling treatment with cobalt-coloured lead glass
by GAAJ-ZENHOKYO Research Laboratory: Dr. Ahmadjan Abduriyim
December 26, 2007
Fissures and cavities filled ruby with lead-based glass started appearing on the market at the beginning of 2004.
Low quality ruby containing many fissures or fractures used to be filled with lead-based glass that has similar RI as the ruby so that the clarity appearance of the stone will improve (photo 1). (Please refer to Gemmology May 2004 and February 2005 issues for more details.)
According to a report by Gem Research Swiss Lab (http://www.swisslab.net), another type of fissures filled ruby with bismuth-based lead has been spread on the market in Thailand since August 2007, and the bismuth lead filling seems to have succeeded in improving the apparent clarity as in lead glass.
An emergency report on a new filling process was released by Prof. Pornsawat Watanakul, who is a technical advisor of GIT laboratory in Bangkok, Thailand, at the LMHC-New York meeting in October 2007. According to the professor, in Chanthaburi region, blue sapphires called "Super Diffusion Tanusorn" are currently circulated widely. This blue sapphire is produced by filling lead-glass that has been coloured in blue by mixing cobalt metal into a corundum (photo 2), and the name "Tanusorn" takes after the name of the person who developed this treatment.
The professor discussed the process of this new treatment with the local heat treaters, and so far gained the following information;
Colourless or brownish Geuda rough stones from Madagascar or Sri Lanka, which have non-basaltic origin, are separated according to their colour, clarity or internal features before heating. At this stage, low quality corundum containing plenty of fissures, fractures or twinning planes are removed to avoid damage by heating. The rejected low-quality rough stones are soaked in strong acid solutions such as hydrogen fluoride, nitric acid, or aqua regia to remove impurities or organic substance on the stone surface. After this chemical cleaning, the pale-coloured corundum is heated together with lead glass that has been mixed with powdered cobalt metal or cobalt oxide. During this process, the lead glass coloured in blue is filled in fissures, fractures or twinning planes.
These fissure filled sapphires in new process are reportedly sold in the local market at a very low price (several dollars per carat).
By courtesy of Prof. Pornsawat Watanakul, we obtained eight pieces of this new type treated sapphire. The features with photos are introduced below.
This new type of treated sapphire displayed blue colour as shown in photo 3-a, but the colour appeared violet-blue or pink-blue under a strong fibre optic light (photo 3-b). The eight pieces tested ranged from 1.400 to 2.892 ct. They were transparent to translucent material with abundant fractures, cracks, and twinning planes developed inside (photo 4).
Their RI, DR and SG were almost identical to those of general blue sapphire and they were inert under both long and short wave UV lights. They displayed vivid red colour typically seen in cobalt-coloured blue stones through a colour filter. On a hand-held spectroscope characteristic three absorption bands were recognised between red and green on a spectrum.
Under magnification, no silk inclusions or colour zone that were generally seen in corundum were observed, but only fluid inclusions or liquid-film inclusions were recognised. In the abundant fractures and cracks, glassy filling material displaying blue to violet colour were readily recognised (photos 5-a, and 5b).
No flash effect was seen but interference colours due to reflection from filling material were observed (photo 6).
Gas bubbles trapped in fractures or twinning planes were also seen. In observation with reflected light, the reflectance ratio of surface-reaching filled fractures was slightly less than that of corundum (photo 7).
To confirm the degree of the filling we performed an X-ray test. Many white streaks in high contrast can be seen, which correspond to distributed surface reaching fracture and twin plane developed in sapphire (photo 8).
Spectral analysis in UV-Vis region showed distinct absorption bands on 630 nm, 580 nm and 540 nm, as well as faint absorptions on 450 nm and 388 nm.
The former absorption bands, which closely resembled spectra of Co-coloured blue spinel or glue glass, were assumed to originate in cobalt (figure 1).
As a result of spectral measurements in infrared region by FTIR, a broad absorption around 2055 cm-1 together with strong absorptions on 2850, 2923 and 2952 cm-1 were recognised (figure 2).
The absorption positions on 2850, 2923 and 2952 cm-1 coincides with those of oil/resin-related substance; however, no absorptions due to oil was recognised in Raman spectral analysis. In the wavelength range over 2500 cm-1 on their spectra, transparency seemed to become diminished towards higher wavelengths. Such characteristic absorption is not observed in ruby of filling treatment with lead glass.
Figure 1: UV-Vis-NIR spectra / blue: absorptions of Co seen in blue sapphire fissures filled with cobalt coloured lead glass; green: blue spinel; red: cobalt glass.
Figure 2: Infrared spectra by FT-IR / red: fissures and cavities filled ruby with lead glass, blue: blue sapphire fissures filled with cobalt coloured lead glass.
After fluorescent X-ray spectral analysis, other than the main element Al or trace elements Ti, V, Cr, Fe and Ga in a host sapphire, elements such as P, Co and Pb were detected from surface-reaching fractures (figure 3).
Contents of Cr and Fe were higher than that in general blue sapphire, and this is assumedly due to foreign material just like P, Co or Pb. Further analysis by LA-ICP-MS was carried to detect light elements or trace elements more sophisticatedly. From the analysis on a surface-reaching fracture that were coloured in blue, elements such as, in order of higher contents, Si, Na, Ca, K, Fe, B, Ti, Li, Cr, Zn, Ba, Ni, Cu, Zr, La, Ce, Sr, Mg, V, Mn and Mo were detected other than P, Co and Pb that were already detected by X-ray fluorescence analysis. Among those elements, Si, Na, Ca, K, B and Li have also been detected in fractures in fissures filled ruby, while P, Mg, V, Cr, Mn, Fe, Ni, Cu, Zn, Sr, Zr, Mo, Ba, La and Ce were newly detected in the blue sapphire called "Super Diffusion Tanusorn" tested here.
Figure 3: Compositional analysis by EDXRF / blue sapphire fissures filled with cobalt coloured lead glass.
These newly detected elements closely resemble those originate in flux (Sodium tetraborate, Lithium metaborate, Sodium di-hydrogen phosphate etc.), which is an additive generally used in heating corundum, but their definite origin remains unknown.
<Expression on the Gem Report>
LMHC immediately discussed the notation on gemmological reports for this new type of treated sapphire at the New York meeting held in October 2007 and confirmed that an item for colouring treatment shall be added to the established wording for fissures and cavities filled ruby.