Dr. Jayshree Panjikar |
K. T. Ramchandran |
Beryllium
Mass Diffusion
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A
new colour diffusion treatment using beryllium has brought
about dramatic changes in the coloured gemstone marketplace
as it changes pale or nearly colourless corundum into
very attractive yellow and orange sapphires. The most
spectacular change with this technique is the alteration
of pale pink stones into hitherto extremely rare orange
padparadscha sapphires. It also turns bluish-red rubies
to very bright pleasing red ones and dark blue sapphires
to bright blue. Unlike previous diffusion treatments which
could only get the colour to penetrate the surface layer
of the stone, the new technique allows the colour to permeate
through the entire stone — right to its very core
if given enough time. The new treatment also works on
rubies that have not responded to heat treatment.
Traditionally, gemstones have been heat treated –
subjected to high temperatures in an oxygen-free or ‘reducing’
atmosphere to intensify their colour. The end of the last
century saw the development of the colour diffusion process.
This involves heat and another material, which diffuses
through the crystal and gives it colour. The foreign atoms
of the added material jump through gaps in the crystal
lattice using the energy supplied by the high heat. Until
now, the colour-giving atoms could only penetrate a little
way into the crystal, therefore only colouring the surface.
But to all intents and purposes it gave the stone a nice
colour. In the 1990s the Thai market was flooded with
blue sapphires that had been treated with the surface
diffusion process.
These stones are easily detected by laboratories by simply
immersing them in a liquid with the same refractive index.
The edges of the immersed stone appear dark, whereas the
entire mass of the stone remains colourless or light coloured
because the colour-producing metallic atoms such as titanium
or chromium are concentrated on the surface.
New
Treatments
Colour diffused ruby. |
In the new beryllium diffusion process,
the gemstones are heated for 100 hours at 1,780°C
in an oxygen-rich atmosphere with a little beryllium.
In the first of two methods, between two and four per
cent chrysoberyl powder is added to borate and phosphate
fluxes. This method only diffuses the colour a little
way below the surface of the stone. In the second method,
the chrysoberyl powder is added to high purity aluminium
oxide (sapphire) powder. The powder method diffuses
the colour all the way through the stone. Sometimes,
chrysoberyl powder is replaced with high purity beryllium
oxide powder and this enables a reduction in process
time to 36 hours.
Orange padparadscha. |
This process produces the complete range
of known gemstone colours plus a few more that are unique. The
low concentration of beryllium renders it undetectable
by most analytical instruments.
Natural unprocessed Montana sapphires
and pale yellows from Madagascar turn yellow or orange.
Some very pale yellows from Madagascar and the bluish
and greenish stones from Songea turn bright yellow or
golden. The ‘colourless’ Sri Lanka sapphire
which results from heat treating certain types of geuda
also becomes yellow, gold, or orange. In some of the
greenish-yellow Australian sapphires from the Subera
deposit, the greenish overtones are removed and the
stones become good yellows or golden. The process also
works on high purity, colourless synthetic sapphires
like the Czochralski-grown ones produced by the Union
Carbide corporation. They develop a golden yellow colour.
Many of the rubies from Songea and Madagascar have a
strong bluish hue component which is replaced with yellow
and the stones become a strong orange when diffused
completely. However if the diffusion is limited to a
surface layer, the yellow surface layer will visually
cancel out or offset the bluish core and give rise to
a very good ruby colour. These stones can, of course
be relatively easily detected. However, by completely
diffusing the stone so that it turns orange and then
subjecting it to simple heat treatment, the yellow overtone
is removed and an optimal ruby colour is achieved.
The same two-step full diffusion and heat treatment
can be used with pink sapphires to achieve padparadscha
orange without a visible diffusion layer. Stones produced
by this two-step method are very difficult to detect.
Colour change wrought
by diffusion. |
Beryllium diffused
padparadscha. |
Difficult to Identify
Diffusion-treated blue sapphires when heated with the
same parameters, get a colourless rim or outer layer
which makes identification difficult. Most identification
centres around inclusions – zircon, glassy discoids
which sometimes contain fern-like feathers and rutile
crystals with blue halos. If there are no inclusions,
detection is very complicated. As the properties of
the treated stones also do not change, no evidence of
treatment can be gathered easily. If the diffusion has
penetrated thoroughly then identification requires sophisticated
techniques like Secondary Ion Mass Spectrometry (SIMS)
and Laser Ablation Inductively Coupled Plasma Mass Spectrometry
(LA-ICP-MS), which can detect the presence of beryllium.
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