The Swiss Gemmological Institute (SSEF) has conducted an extensive research project to establish more specific criteria to detect low-temperature heat treatment of rubies from the deposits near Montepuez in Mozambique which often did not show the microscopic features that commonly characterise heat treatments.

Detailed results of the study will be published soon, the organisation said in a release.

Giving a background to the study, the SSEF said that since the discovery in early 2009 of the ruby deposits near Montepuez, there have been an impressive number of exceptional-quality stones, including iconic gems such as the Rhino Ruby (22.04 cts), the Scarlet Drop (15.95 cts) and the Eyes of the Dragon (a pair of rubies pair weighing 11.23 cts and 10.70 cts), all of which were analysed by the SSEF. At the same time, there has also been evidence in the market that some lower-quality rubies from these deposits are sold after having undergone heat treatment.

The number of such fine quality rubies that have come onto the market after having undergone so-called "low-temperature heating" (below 1000 °C) has increased in recent years. The aim of the treatment, the lab presumed, was to enhance the colour slightly, by reducing subtle purplish zones which are sometimes present in rubies from this location.

Gemmologists, however, faced a challenge as only a portion of the treated rubies showed the microscopic features easily recognisable by an experienced gemmologist, while others showed no or nearly no heat-related transformations of inclusions.

In this context, SSEF conducted a study of more than 200 unheated and heated rubies (rough and faceted stones) from Mozambique. In combination with close microscopic examination of the samples, specific infrared spectroscopy (FTIR) peak features were analysed in order to determine additional distinguishing criteria. For decades, FTIR has been considered a useful tool to assist in the detection of heated rubies and sapphires because of the presence of absorption peaks related to structural hydroxide (OH-) in heated stones, the lab said.

The study showed that, rather than using individual peaks to determine if a ruby has been heat-treated or not, the focus should be on peak patterns.

“Due to the challenges associated with detection of low-temperature heat treatment of rubies from Mozambique, we felt that more scientific research was necessary to shed light on more specific detection criteria using FTIR," said Dr. Michael S. Krzemnicki, Director of SSEF. "We are confident that this and other research will contribute to consumer confidence in the beautiful ruby material coming from Mozambique."

Pic caption: Slightly purplish zone in ruby from Mozambique
Photo Courtesy: M.S. Krzemnicki, SSEF