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HEAT
TREATMENT OF SAPPHIRE
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| In
recent months, several blue sapphires
submitted to the GIA Gem Laboratory
for identification have shown
evidence of high-temperature heat
treatment, as well as “billowy”
blue color concentrations when
viewed with immersion. Photo by
Elizabeth Schrader. |
| Since
the advent of beryllium diffusion
treatment and the trade’s awareness
of the color changes that certain
chemical elements can achieve
when diffused into the lattice
of a ruby or sapphire, gemologists
from around the world have been
on the lookout for any new developments
in the heat treatment of rubies
and sapphires. What appears to
be the latest development, first
noticed early in 2003, has taken
the form of blue sapphires with
unusual color concentrations.
To date, we have examined dozens
of sapphires that showed these
unusual color concentrations as
well as clear evidence of high-temperature
heat treatment. With immersion,
they revealed irregular or “billowy”
blue color concentrations, surrounded
by pale blue or colorless areas
that extended to the surface (figure
1). At the boundary between the
inner and outer color concentrations,
two features appear to be indicative:
(1) a thin, near-colorless zone
that parallels the contours of
the inner color concentration;
and (2) a scalloped edge that
is bordered by distinctive growth
and color zoning.
There has been
considerable speculation about
the process used to achieve
the unusual color concentrations
observed in this material. Although
we have seen several known Be-diffused
blue sapphires, to date no significant
amounts of beryllium have been
found in these particular stones.
Another theory that has received
considerable attention involves
the partial dissolution of the
original sapphire during this
“new” heating process and subsequent
regrowth (i.e., of synthetic
sapphire) over the remaining
original stone. |
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| Prior
to heat treatment, the samples
were placed in an alumina crucible
without a fluxing agent. Photo
by Matthew Hall. |
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To investigate this new treatment
further, GIA researchers Christopher
P. Smith and Matthew Hall recently
traveled to Sri Lanka, where they
attended experiments by Punsiri
Tennakoon of Punsiri Gems, the
man who developed this process.
The purpose of their trip was
to learn more about the procedures
and conditions being used, as
well as to try to determine the
mechanism that might be responsible
for the particular gemological
features that have been observed.
Tennakoon's
heat treatment experiments used
several samples he supplied
as well as reference material
that had been pre-analyzed by
GIA (figure 2). The heat treatment
procedure (which is proprietary
to Tennakoon) had a dramatic
effect on the color of most
of the samples (figure 3), and
initial observation suggested
that the unusual color concentrations
indicated above were produced
in more than two-thirds of them
(figure 4).
On
the basis of detailed microscopic
examination of the samples after
treatment, Smith and Hall determined
unequivocally that no synthetic
sapphire growth had taken place.
Furthermore, the researchers
did not witness the use of any
fluxing agents during the heating
experiments, and no evidence
of flux was present on or around
the samples brought back to
the United States for further
analyses. At present, we cannot
state conclusively the role
that lattice diffusion plays
in this peculiar color phenomenon
or which elements may be involved.
However, we are continuing our
research into the exact mechanisms
responsible for these unusual
color concentrations, and will
release our findings to the
trade as they become available. |
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| When
viewed in the proper direction,
the heated half of the sample
in figure 3 showed the same kind
of unusual blue color concentration
that has been noted in several
blue sapphires submitted to the
laboratory in the past several
months. Photo by Matthew Hall.
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| This
9.74-ct. sample (approximately
16.1 × 10.3 × 6.3 mm) was sawn
in two just prior to the heat
treatment experiment, and the
half on the left retained as a
control. Note the dramatic change
in color in the half on the right
following treatment. Photo by
Matthew Hall. |
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