Cobalt-doped glass-filled sapphires: An update

The history of glass-infilling in blue sapphire is reviewed, along with a detailed description of the latest generation of this treatment, which was developed by Tanusorn Lethaisong in Chanthaburi, Thailand. Stones may be identified by reference to their inclusions, lack of pleochroism and red reaction in the Chelsea filter. Spectra and other advanced tests further characterize and separate these stones from natural and synthetic sapphire, as well as heated and bulk diffused sapphire. Introduction In September of 2007, five faceted stones and a few rough samples were submitted to the Bangkok lab of the Gem & Jewelry Institute of Thailand (GIT) by a gem treater who informed the lab that they represented a new type of treatment. The name applied to this treatment at the time was Super Diffusion Tanusorn, named after Tanusorn Lethaisong, the Chanthaburi, Thailand treater who developed it (GIT, 2007). In reality this treatment did not involve diffusion, but was simply an infilling of cobalt-coloured glass into a highly fractured corundum, dying it blue. The starting material for this treatment was said to be low-grade near-colourless corundum from Madagascar or Sri Lanka (Abduriyim, 2007). Initial finished samples were of low quality, being semi-translucent. Five years later, in May of 2012, GIT’s lab received two unusual blue stones for identification (Leelawatanasuk, 2012). One was a 6.62-ct cabochon, with the other Figure 1. Two blue stones weighing 7.57 ct (left) and 6.62 ct (right) submitted to GIT for testing in May 2012. Photo by Warinthip Krajae-Jan. being a 7.57-ct stone that was faceted on the crown and cabochon cut on the pavilion (Figure 1). It was said that the stones were blue sapphires heated in the conventional fashion. Refractive index and specific gravity values were typical for sapphire within the degree of error. However microscopic examination revealed features that were decidedly not natural. While some features (included crystals, lamellar twinning) were typical of natural sapphire, the stones were riddled with fissures filled with a rich blue substance that stood out strongly against the otherwise near colourless sapphire. Indeed, so strong was the blue substance in the fissures that it strongly coloured what would otherwise be near colourless sapphires. In effect, the stones were dyed blue. Since that time, a number of additional pieces have been examined. Effectively these stones represent a refinement of the original Tanusorn treatment. What follows is a discussion of the general properties and key identifying features of this new type of treated sapphire. General properties The gemological properties for these stones are generally consistent with natural sapphire, with a few important exceptions. Specific gravity values were slightly elevated in some specimens. But more importantly, all of the stones tested showed a strong red to orange-red when viewed with the Chelsea filter. Both natural and synthetic blue sapphires show no red. In addition, examination with the dichroscope revealed a lack of pleochroism. This is because the rich colour of the stones comes from the cobalt colourant in the glass, rather than from the sapphire structure itself. Together, the Chelsea filter reaction and lack of pleochroism allow easy separation from both natural and synthetic sapphire, including sapphire enhanced by both standard heating and titanium bulk diffusion. Figure 2. Two first-generation cobalt-doped glass-filled sapphires (top row; 3.49 & 3.37 ct), along with eight latest-generation cobalt-doped glass-filled sapphires ranging from 1.50–2.25 ct each that were tested as part of this study. Note the far lower clarity of the first-generation stones. (Photo: Wimon Manorotkul)