gems, corundum, treatments.6, synthetic corundum.7, titanium diffusion

Since 2015, a large quantity of a new blue diffused synthetic sapphire has appeared in the Chanthaburi market. It is interesting to compare its gemmological properties with those of a blue diffused natural sapphire that came onto the market in 2012. In immersion, both types of blue diffused sapphires showed the “spiders’ web effect” — a cut-related colouration pattern — and rather high titanium content on the surface of the samples, both effects being diagnostic evidence of Ti-diffusion treatment. The synthetic test stone, however, also displayed strong chalky blue fluorescence to SWUV radiation (also in the DiamondView™), the presence of curved bands, minute particles or gas bubbles under a microscope, very low Ga contents, and no Fe³⁺-related absorption peaks on the UV–Vis spectra. In contrast, the natural one showed the occurrence of tension discs, altered solid inclusions and altered fingerprints, often with colour concentration along healed fractures, cavities seen under the microscope, significant Ga content, and some Fe³⁺-related absorption peaks on the UV–Vis spectra. The thickness of the blue colour rim of the diffused natural stone was apparently much thicker than that of the diffused synthetic one. The reason is that it is the penetration depth of titanium that controls the colour rim thickness of the natural stone, while it is the penetration depth of iron that controls that of the synthetic one. Because of the difference in colour rim thickness and the repolishing process, the “spiders’ web effect” in the natural stone is less pronounced, while that in the synthetic one is more obvious. The formation of the blue colour rim and multi-element diffusion found in both synthetic and natural sapphires are also discussed here.