Ti-diffusion treated sapphire cabochons

by Dr. M.S. Krzemnicki, first published in Facette 28 (May 2023)

Figure 1: Five Ti-diffusion treated sapphires examined at SSEF. Photo: SSEF.

Titanium diffusion treatment is known since decades: it can lead to a surface-related blue colour in corundum. Basically, this treatment is a heating process requiring high temperatures in combination with titanium (oxide) as an additive. During the treatment, titanium atoms penetrate into a superficial layer of corundum. By pairing with bivalent iron (already present as a trace element in the corundum structure), a strong blue colour is achieved by intervalence charge transfer (Ti4+ – Fe2+ IVCT) within this superficial layer in the corundum.

The artificial creation of colour by adding an element (in this case titanium) from an external source has to be fully disclosed throughout the trade and to the final consumer (CIBJO). Even more so, as this sapphire-blue colour layer at the surface of corundum is commonly only a fraction of a millimetre thick and would be completely lost during re-cutting.

Figure 2: Disc-like extension features in one of the described cabochons as proof of heating at high temperatures during the Ti-diffusion treatment. Photo: M.S. Krzemnicki, SSEF.
Figure 3: Diffused blue zone and dark blue spot around small cavity filled with glassy residue in one of the Ti-diffusion treated sapphire cabochons. Photo: M.S. Krzemnicki, SSEF.
Most of the sapphires submitted to SSEF are unheated and of high quality. It was thus a rather rare encounter when we received a series of 5 cabochons (Figure 1), all showing tell-tale signs of surface diffusion by titanium. For faceted stones, the detection of such surface-diffusion treated corundum is usually rather straightforward as they show facet- related variations in blue colour. For the described stones, however, this easy detection was not possible, as no facets were present on these cabochons. Still, we observed many newly healed fissures and characteristic discoid features (“atoll-structures”, Figure 2) as a proof of high temperature heating. And the dark blue concentrations at small indentations and blue diffusion rims along flux-filled channels (Figure 3) clearly revealed that these stones had been diffusion treated with titanium stemming from an external source. This was also confirmed by trace element analyses on one of the dark blue spots at the surface. It revealed excessive titanium concentration about 1000 x higher than ever found in sapphires coloured naturally by the same trace elements iron and titanium.

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