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Unraveling
an Opal's Play of Colour
by Guy Borenstein, FGA EGG |
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One
of the trickiest gems to evaluate is an opal. This beautiful
gem displays a distinctive phenomenon of coloured flashes, which
reflect from the stone which it is rotated. The effect is called
"Play of Colour," and it is a term that well explains
the experience.
The
visual effect of the opal is a result of its physical structure,
which involves silica sphres of about 150 to 300 nanometres
in diameter in hexagonal or cubic close-packed lattices. They
produce the internal colours through the interference and diffraction
of light passing through the stone. As the opal is turned, the
lightis affect differently, and so is created the "Play
of Colour".
While
most gems are graded using the classical factors -- colour,clarity,
weight, etc,--Play of Colour is An additionalfactorthat must
be considered with Opal. How is this Done? |
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The
qualIty of the Colour depended on several characteristic,
these acan be the colour of the flashes, their dispersion
over the gem, the pattern they produce, and more. It is complicated
to determine and demands an essential quality in the grader
- experience.
In order to visualize
and better uderstand this phenomenon, we decided to use the
GemeProTM Sampler to inspect the play of Colour
effect of an opal gem. Since the Sampler was designed to define
the body colour of gems, we didn't actually know how it would
deal with the challenge this unique effect posed. However,
we believed we could useful information from the flashes that
would be of assistance in understanding itsgrading method.
For this research,
we received from L.Allen Brown of All That Glitters (http://www.atggems.com)
images of a magnificent 8.89 ct, oval cabochon-shaped Ethiopian
opal (Figure 1), which was described as displaying a "massive"
play of colour effect, in a pin fire pattern. The gem was
a good candidate, as it shows the effect with several different
colours distributed all over the dome. |
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We
used a face up image of the opal and scanned it using the
Sampler. Instead of retrieving an average body colour (which
is useless if the flashes are part of the calculation), we
recalibrated the Sampler sensitivity to be as twice sensitive
to be as twice sensitive as it regularly is. By doing that,
we were able to provide the main colour groups and their ratios
within the overall appearance of the gem. From the resultant
list, we eliminated the body colour hues and were left only
with the coloured flashes.
As seen in Figure
2, the main colours that the sampler idetified were Red(5.88%),
orange(0.37%), Yellow(0.37%),, Green(1.48%), Blue (1.48%)
and violet(0.37%).
The
results demonstrate the visibility of the effect and its intensity
- almost 10% of the gem reflects Play of Colour fllashes,
Considering the fact that the analysis was carried out on
a static image, the amount of flashes in reality may even
be higher. Moreover, we can see that effect is not limited
to a few colours, but rather their combinations cover the
entire spectrum.
Looking at the Colour
DNA (FGigure 2, left hand side), it is clearly evident that
the coloured reflections are distributed all over, which means
that they are spread over te entire dome. This beautiful opal
has indeed earned its play of colour effect's description
as "massive" |
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