June 8, 2017 in articles

DIFFUSION-INDUCED BLUE COLOUR AND ASTERISM SAPPHIRE

By Tay Thye Sun and Loke Hui Ying

(Published in the Journal of Gemmology, 34(7), 2015, pg 576 – 577)

In April 2015, a client submitted a bright blue star sapphire weighing 13.69 ct and measuring 13.85 x 11.89 x 10.28 mm (Figure. 28). The crown was polished into a typical domed cabochon, while the pavilion was faceted so that it reflected light like a faceted stone. This cutting arrangement is only rarely seen in natural star sapphires. The six-rayed star had straight ‘arms’ that were sharp and bright under a point light source, and the stone exhibited a double star when photographed from an angle.

The crown gave a spot RI reading of 1.76, and the faceted pavilion yielded RIs of 1.76 – 1.77 (birefringence of 0.010). The gem was inert to UV radiation. Viewed with magnification, the stone had small ‘fingerprint’ inclusions (e.g. Figure. 29) that indicated a natural (not synthetic) origin. However, there were no prismatic needle-like inclusions that are typically associated with asterism in sapphire. Instead, at higher magnification (20x – 30x), the asterism was seen to be caused by wispy ‘silk’ or fine fur-like inclusions that were present just under the surface (Figure. 30). Such inclusions have been ascribed to diffusion treatment in synthetic star sapphire (e.g. ‘AIGS finds more stars’ 1994; Kammerling and Fritsch, 1995; Singbamroong et al, 2015). When the stone was immersed in eucalyptus oil and viewed with diffused lighting, the pavilion showed inconsistent colouration between various facets and colour concentrations along facet junctions (Figure. 31), proving that the gem was repolished after diffusion treatment (e.g. Kane et al, 1990).

We concluded that this was a natural sapphire that had been diffused-treated to induce its colour and asterism. Although some previously documented diffusion-induced star sapphires were synthetic in origin (‘AIGS finds more stars’ 1994; Kammerling and Fritsch, 1995; Singbamroong et al, 2015), gemmologists should remember that natural as well as synthetic starting materials may be used for this type of treatment.

 

Figure. 28: Both the asterism and blue colour of this 13.69 ct sapphire are the result of diffusion treatment. Photo by Tay Thye Sun.

Figure. 29: This ‘fingerprint’ inclusion found under the dome of the star sapphire displays subtle interference colours. Note the hazy appearance of the stone’s surface and minute pits that resulted from the diffusion process. Photo by Tay Thye Sun; magnified 35x.

Figure. 30: Wispy ‘silk’ is responsible for the asterism in the sapphire. Photomicrograph by Tay Thye Sun; magnified 30x.

Figure. 31: Inconsistent colouration between various facets and colour concentrations along facet junction of the pavilion prove that the sapphire underwent diffusion treatment and was then repolished. Photomicrograph by Tay Thye Sun; magnified 20x

 

Acknowledgement: The authors thank Mr. Yu Xiong for bringing this sapphire to their attention.

References:

AIGS finds more stars, 1994. Jewellery News Asia, June 118, 74.

Kammerling RC. And Fritsch E, 1995. Gem Trade Lab Notes: Synthetic sat sapphires with an unusual color. Gems & Gemology, 31(1), 5758.

Kane R.E., Kammerling, RC.,Koivula J.I., Shigley J.E., and Fritsch E., 1990. The identification of blue diffusion-treated sapphires. Gems & Gemology, 26(2), 115-133, http://dx.doi.org/10.5741/gems.26.2.115.

Singbamroong S., Ahmed N. and Al Muhairi N., 2015, Gem News International: Synthetic Sapphire with diffusion-induced color and star. Gems & Gemology, 51(2), 203-205.

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