Cobalt-Doped Composite Sapphires: A Blessing or a Curse?
Gem mining is expensive and it is economically important to be able to enhance stones of lower quality. Treatments allow increasing the supply of gemstones making them available at affordable prices. Treated gemstones have been flooding the markets. The potential damage is a loss of consumer confidence. Are cobalt-doped composite sapphires a blessing or a curse? Only time will tell.
Treatments of corundum are meant to enhance transparency, color, and stability, or to create optical effects. Treatment options include: fissure filling with oil, dying, heating, irradiation, surface diffusion, heating with borax, lattice diffusion of beryllium, lead glass fissure filling, and cobalt glass fissure filling.
Glass infilling in blue sapphire was developed by Tanusorn Lethaisong in Chantaburi in 2007, and in May 2012, a “new” type of cobalt-colored sapphire became available in the Bangkok market. Christopher P. Smith, in his recent article (“Cobalt-Colored Composite Sapphires,”InColor, Spring 2013,No. 22, pp. 32−37), explains what the fuss was all about.
He discovered the sapphires to be a composite of natural, low-grade corundum and a high-lead glass that induces blue coloration and enhances clarity. The result is a sapphire with a medium-saturated color.
These stones are not a challenge for a trained gemologist to identify. Gemological properties include readily evident color concentrations in surface reaching fissures and cavities, innumerable spherical to flat contraction bubbles, a “flash effect,” naturally occurring crystals, and short, fine unaltered rutile needles. The intact rutile indicates that the treatment takes place at relatively low temperatures.
Figure 1: Spherical to flattened contraction bubbles, irregular black reflective particles, “flash effect,” and color concentrations are observed in these glass-filled sapphires. Photo courtesy of Christopher P. Smith, InColor.
The refractive index of the lead glass is similar to that of the host corundum, but the glass has a lower hardness. As a result, the polished surfaces of the stones reveal a mosaic network of narrow to thick tracks of glass. The stones glow bright red with the Chelsea filter. Using a standard hand-held spectroscope, a series of three distinct absorption bands positioned at approximately 528, 588, and 635 nm readily identify cobalt to be the main coloring agent. Such cobalt-related absorption bands do not occur naturally in sapphire.
Figure 2: Broad absorption bands centered at approximately 528, 588 and 635 nm identified the coloring agent as cobalt. Photo courtesy of Christopher P. Smith, InColor.
Infrared spectroscopy recorded absorption bands at approximately 2290, 2620 and 3490 cm-1. These bands are related to the glass component of the composite sapphires. These specimens require special care, as standard manufacturing and bench jeweler practices, as well as household products can readily cause damage by etching the glass. The product has been classified as “composite sapphire” by the AGL (American Gemological Laboratories). An in-depth study by Leelawatanasuk et al. (“Cobalt-doped glass-filled sapphires; an update”, The Australian Gemmologist, ,Vol. 25, No. 1, 2013, pp. 14–20) provided additional information on the properties of the cobalt-colored composite sapphires. The report noted slightly elevated specific gravity in some specimens and a lack of pleochroism.DiamondView images showed a chalky blue fluorescence caused by the glass-filled fissures, and filled fractures appeared white on X-ray images.
The value of a gemstone depends on its inherent beauty and rarity. The artificial modification of a gemstone greatly reduces the rarity and therefore lowers the value. Treatment of these “composite sapphires” requires special care to retain it, or else the benefit of the treatment will be lost over time. The Federal Trade Commission therefore requires proper disclosure to buyers.
