Jeweller - February 2022

REVIEW
Gems
Part II: Synthetic Diamonds
Above: Swarovski Created Diamonds
With the success of growing gem-quality
diamonds at a reasonable cost, the next
step for the synthetic diamond industry was
to achieve larger sizes, improve quality, and
produce an array of colour options to offer
consumers.
Following production, synthetics are
often assessed to determine treatment
methods that will improve colour and
achieve a higher price.
These treatments are not exclusive to
synthetic stones, low quality natural
diamonds often undergo the same
processes.
The best course of action will depend
on the characteristics of the diamond
to begin with and the desired result.
Though rare, it’s important to note that
clarity enhancement has also been
recorded in synthetic diamonds.
Colour treatments, unlike clarity
enhancement - a cosmetic treatment
altering the appearance of the diamond -
alter a diamond’s atomic lattice.
Given colour is imparted by the defects
present in the lattice, the goal of these
treatments is to add defects, remove
them, or a combination of both.
One of the most common treatments
that Chemical Vapour Deposition (CVD)
synthetic diamonds undergo is High
Pressure High Temperature (HPHT)
treatment. CVDs are usually brownish in
colour and therefore worth less than the
colourless alternative.
This colouring is a result of the much
faster growth times of CVD synthetics
compared to HPHT produced synthetics.
When diamonds grow too quickly,
dislocations happen in the lattice and
these brown tones begin to appear.
This is of no concern to the
manufacturers, because HPHT treatment
can take only minutes to perform.
Growing stones faster then HPHT treating
them to remove the brown will still result
in a nice colourless diamond and is more
cost effective than manufacturing the
diamond at a slower pace.
Altering the colour of CVD synthetics
doesn’t end there – additional or
alternative treatments can produce
blues, pinks, yellows, and more. Again,
the resultant colour will depend on the
atomic structure and any defects present
in the diamond to begin with.
Yellow colouring like the ‘cape’ yellow
colours seen in natural diamonds can
also be produced with HPHT treatment.
In other cases, following HPHT treatment
with irradiation and low-temperature
annealing can produce a range of pink,
orangey-pink, purplish-pink, and even
red hues.
The colours produced can be beautiful
and even similar to the colours of the
rare Golconda-type natural, untreated
pink diamonds.
Post-growth irradiation alone can
result in blue and blue-green hues.
This irradiation treatment creates the
same GR1 (general radiation 1) defect
present in natural green and greenishblue
coloured diamonds – a problem in
itself for determining whether the origin
of colour in a diamond is natural or
introduced.
How are
synthetic
diamonds
coloured?
PINK
HPHT treatment
and / or irradiation +
annealing
RED
HPHT treatment
and / or irradiation +
annealing
GREEN
Irradiation or nitrogen +
boron together or nickel
BLUE
Irradiation or boron
PURPLE
HPHT treatment
and / or irradiation +
annealing
BROWN
Dislocations
YELLOW
Nitrogen
ORANGE
Irradiation + annealing
or nitrogen
BLACK
Inclusions or irradiation
Unlike CVD synthetics that often owe
their colouring to post-growth treatment,
it’s more common for HPHT synthetic
diamonds (that is, diamonds grown by
the High Pressure High Temperature
process) to be coloured ‘as-grown’.
Steps such as removing or introducing
nitrogen in the growth chamber itself
can produce the desired colour in HPHTs,
rather than treating the stone postgrowth.
The pink, purplish-pink, and red colours
seen in HPHT synthetic diamonds
are induced by irradiation and lowtemperature
annealing treatments
post-growth. Green hues can also be
a result of irradiation, though it is not
always the cause.
Given these diamonds are grown in a
laboratory to begin with, there is less
emphasis on determining whether colour
in synthetic diamonds is ‘as-grown’ or
introduced by further treatment, post
manufacturing.
On the other hand, determining the
origin of colour (whether treated or
natural) in natural diamonds can be one
of the most difficult and important tasks
gemmologists currently face.
Mikaelah Egan FGAA Dip DT
began her career in the industry at
Diamonds of Distinction in 2015. She now
balances her role at the Gemmological
Association of Australia with studying
geology at the University of Queensland.
Visit instagram.com/mikaelah.egan
For more information on gems and
gemmology ,go to www.gem.org.au
February 2022 | 35