New research on Ruby by SSEF
Studies
methods of detecting
low-temperature
heated rubies from Mozambique
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 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 Swiss Gemmological Institute SSEF.
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.
Only
a portion of the rubies that have been subject to this relatively
low-temperature heating procedure show the microscopic features that commonly
characterise heat treatments, which would be easily recognisable by an
experienced gemmologist (GĂĽbelin & Koivula 2008, Pardieu 2013, Krzemnicki
2015).
Others
show no or nearly no heat-related transformations of inclusions (see also
Pardieu et al. 2013, Saesseaw et al. 2018). It is a situation that challenges
gemmologists and gemmological laboratories, and also, ultimately, the trade.
To
meet the challenge, the Swiss Gemmological Institute 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. 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 (Moon & Philips
1991, Smith 1995, Beran & Rossman 2006, Saesseaw et al. 2018).
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 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."
Caps:
Figure 4: FTIR spectra of unheated rubies from Mozambique (in green, below) and
heated rubies (in red, above). The detailed studies clearly show the need for a
combined microscopic and FTIR approach, along with analysis of FTIR peak
patterns rather than presence or lack of individual peaks.
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