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 to begin with.
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.
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.
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