New and additional criteria 
to detect low-t heated corundum

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

Figure 1: “Hot” pink sapphire from Ilakaka, Madagascar and clusters of tiny colourless zircon inclusions characteristic and important markers for such pink sapphires. Photo: SSEF.

Heat treatment of ruby, sapphire and other colour varieties of corundum is one of the main issues for the gem trade. As such, detecting such treatment is a major task of gemmological laboratories issuing gemstone reports. Unfortunately, and especially lately, the detection of such heating is not always straightforward. This is especially the case when the applied heat is of rather low temperature (< 1000 °C). Such treatment is commonly known in the trade as low-T heating (see also SSEF Press Release from September 2018).

At SSEF, we have seen in the past few months an increasing number of pink sapphires (Figure 1) and other corundum varieties (e.g. ruby, purple sapphire) on which such low-T heat treatment had been applied with the aim to slightly shift the colour to a more attractive and vivid hue.

Unfortunately, traditional detection methods based on detailed microscopy, reaction to ultraviolet light, and infrared spectroscopy is in certain cases not conclusive to tell whether such a stone has been heated (at low-T) or not. Based on numerous heating experiments at SSEF and using new and highly sensitive instrumentation and analytical protocols, we have been able to explore new scientific criteria to separate unheated pink sapphires (and other corundum varieties) from heated ones. These new criteria are mainly based on detailed Raman spectroscopic analyses of inclusions, specifically those which su er major structural transformations by a heating process (Figure 2).

Figure 2: Automated curve-fitting developed at SSEF to analyse the shape of the main Raman peak of zircon inclusions in corundum and the effect of heating on the peak shape of zircon inclusions within this sample. Diagram: SSEF

Consequently, we have recently adapted our testing protocol and are now able to get more unambiguous and conclusive results even for cases which were considered challenging or inconclusive before. On the other hand, however, this also means that considerably more analytical time and e ort needs to be invested in the laboratory in our daily operations when testing such stones.

Having said this, it is also important to understand that any result on a gemstone report at the very end reflects the scientific knowledge at the time of examination. As the results of our current research show, scientific gemmological knowledge is constantly evolving. Thus, in certain cases this may result in a reassessment of a previous conclusion because of new and previously unknown or unavailable scientific criteria. Neglecting this fact cannot be a solution for the trade nor for gemmological laboratories.

To conclude, the Swiss Gemmological Institute SSEF applies rigorous testing procedures and is constantly investing in research and state- of-the-art instrumentation so as to be able to identify all sorts of new developments related to gemstones and pearls. We consider it also our mission to inform the trade openly and timely about new research findings and testing criteria, always with the aim to protect the gem trade and to maintain the confidence of the public in gems.

A detailed scientific publication about our latest findings is in preparation and will be published in the Journal of Gemmology.

Want to learn more about sapphires?

Start your journey to becoming a sapphire expert with our free online course “Introduction to sapphires”. Learn all about sapphires. Their fascinating history, how they form, where they come from. Learn about all the different origins of sapphires and their treatments. Take this course as an introduction to the wonderful world of sapphires.