The Swiss Gemmological Institute (SSEF) has said it has worked on techniques to detect rubies from the Montepuez deposit in Mozambique that have been heat-treated at temperatures under 1,000℃. Only a few of these rubies display the characteristics of heat treatment that help gemologists detect them.
An SSEF press release noted that since their discovery in early 2009, the ruby deposits near Montepuez in Mozambique have produced an impressive number of exceptional-quality stones, including iconic gems such as the Rhino Ruby (22.04 carats), the Scarlet Drop (15.95 carats) and the Eyes of the Dragon (a pair of rubies weighing 11.23 and 10.70 carats), all of which the lab has analysed. But from the very beginning, there has been evidence in the market of lower-quality rubies from Mozambique that have been heated with or without a flux (borax), resulting in healed fissures with residue, and in some cases heavily-fractured material that has been lead-glass filled.
In more recent years, an increasing number of fine quality rubies from Mozambique have come onto the market, after having undergone so-called "low-temperature heating" (below 1000 °C). Presumably, the aim of this treatment is to enhance the colour slightly, by reducing subtle purplish zones which are sometimes be present in rubies from this location.
To meet the challenge, the SSEF has conducted an extensive research project to establish more specific criteria to detect low-temperature heat treatment, studying 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.
The SSEF's FTIR 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. The results of this study will be published in the coming months.
"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 SSEF Director Dr. Michael S. Krzemnicki. "We are confident that this and other research will contribute to consumer confidence in the beautiful ruby material coming from Mozambique."