DISTINGUISHING
BETWEEN UNTREATED AND HEATED
SAPPHIRE
Lapidaries in Sri Lanka who
deal in sapphire can determine
whether a stone is untreated
or heat-treated by looking
at the finished stone's appearance.
Differences in the transparency
and color result in difference
in beauty. Through many years
of experience, they can recognize
an untreated stone.
Simply
speaking, sapphire gets its
blue color from the elements
titanium and iron. If these
elements are present in a pale
or colorless stone, heat treatment
causes a chemical reaction
that brings out a deeper blue
color.
The high temperatures
used to change the color of
corundum often have a large
effect on the gemstones' inclusions
as important clues in determining
if a gemstone has been treated.
However, interpreting the meaning
of inclusion is difficult even
for professionals, and other
instrumentation is used to
confirm whether a sapphire
has been treated or not. |
Characteristics
of Heated Sapphire
The temperatures used during
the heat treatment of corundum
are often high enough to melt
or otherwise seriously damage
inclusions of other minerals
such as rutile, apatite, or
hematite. Melted crystals like
the "snowball" inclusions
discussed on here are often
seen in heat-treated sapphire
and rubies.
Another type of
inclusion seen in heated corundum
is the disc-like fracture.
These are tiny internal fractures
that form when an included
crystal bursts. The melted
crystal may enter the break
and solidify, often leaving
a fringe (as shown in the center
of the photograph to the right).
These kinds of inclusions offer
us a glimpse of the intense
nature of the heat treatment
process. |
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Silk Inclusions
Corundum may contain fine, needle-like
crystal inclusions of rutile (titanium
oxide). These are commonly called "silk" because
of their thread-like appearance.
Often occurring in sets of parallel
lines that intersect at
60 angles
as shown in the photograph, silk
inclusions range in size from those
that are eye-visible, to those
that visible with a 10-power loupe,
to those that require 30-power
magnification under a microscope.
Silk inclusions can be seen in
about 30 percent of the untreated
sapphires from Sri Lanka. They
often melt during heat treatment,
so the presence of silk suggests
that a stone has not been heated.
During treatment, titanium from
the silk inclusions may also cause
a blue color to appear. If enough
silk inclusions are present, an
eye-visible band of color will
appear in the stone. |
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Silk Inclusions
Untreated
Magnification : 63x |
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Disk-Like Fracture
Inclusions
Heated
Magnification : 20x |
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JUDGING QUALITY
The first step is to confirm that
a sapphire has a beautiful and
balanced mosaic pattern when viewed
from the table (face-up). A well-balanced
mosaic pattern is the basis for
determining the beauty of a faceted
gemstone, regardless of whether
it is untreated or heat-treated,
or the tone of its color.
Many
stones, especially untreated sapphires
from Sri Lanka, are fashioned with
an extremely poor make and shape
in order to preserve as much of
the rough material as possible.
There is no need to become anxious
about it, but sellers should establish
standards for total depth-the ratio
of the depth of the stone to its
width-to be within a certain range
(for example, 50 to 80 percent).
Such standards will serve as a
basis for making buyers feel more
comfortable. It is also important
not to be critical of imperfections
to the point where one would pass
up a beautiful and appealing stone.
Other than large inclusions or
color patches that are obvious
to the unaided eye, imperfections
that do not strongly affect beauty
are best viewed as minor details.
The presence of fine silk inclusions,
with their characteristic thread-like
appearance, is proof of natural
origin, and, if they are intact,
an indication of non-treatment.
As long as these silk inclusions
do not adversely affect a gem's
beauty, they can even be thought
of as a positive presence. |
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