Heat treated Sapphire in pressure environment

Lisa Greggio1, Muna Al Bulushi2, Maya Musa3

1 GIG Laboratory Director
2 GIG Senior Gemology Analyst
3 GIG Research Department Director

Fig.1The 5.10 ct Oval mixed cut Sapphire analyzed by GIG Team.

Recently, the GIG Laboratory team analyzed an oval mixed cut Natural Corundum, Sapphire variety, weighting 5.10 ct [Fig. 1]. The stone has been graded as Blue color and Transparent.
By the optical microscope the features of the heat treatment have been easily recognized: pinpoint inclusions, strongly altered crystals presenting melted and/or “glassy” aspect, surface-reaching fractures.
In GIG one of the high-tech analyses routinely performed, in order to verify possible heat treatments in corundum, is the FTIR spectroscopy. This test is typically performed focusing on the OH defects [Musa & Greggio, 2014; Scarrat, 2017]. In figure 2 the Absorption spectrum of the Sapphire analyzed by GIG, with the relatively OH range detailed, has been reported. It must be noted the presence of faint 3309 cm-1 band, characteristic of single OH stretching in the corundum structure [Rossman, 2006] and considered as a common feature of heat treatment in metamorphic sapphires, plus a stronger broad band centered about 3040 cm-1. This broad band series is reported for Sapphires treated in extreme condition, like high-temperature and high-pressure environment [Peretti, Musa et al., 2018], obtained by the application of the HPHT systems, originally developed for the diamonds treatment.
This is a relatively recent treatment: the first sapphires appeared it in the trade around 2015/2016 and its nomenclature as well as effects on the stones are under discussion between the labs. The Final color and aspect of the stone nowadays are considered totally stable in time.

Fig. 2 FTIR Spectrum carried out without any sample preparation thanks to the praying mantis diffuse reflectance accessory. The broad band centered about 3040 cm-1 and detailed in the box corresponds to the defects vibrational modes observed in several cases on HPHT treated Sapphires

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