Argyle’s diamond-grading system
In order to accurately grade brown, pink, and blue diamonds from Argyle, Rio Tinto developed its own grading system, which is substantially different from Gemological Institute of America’s (GIA’s) and thus, very difficult to correlate grade to grade.
The grading of these diamonds at Rio Tinto’s Perth facility is very strict, often one clarity grade lower than other international labs. The Argyle colour system is very specific, using more categories than GIA. However, the only drawback is that it is made specifically for Argyle-mined diamonds and is difficult to apply to coloured diamonds sourced elsewhere. Pink diamonds with orangey modifiers from Brazil are one example; the Argyle system simply does not have an option for this colour.
Within the Argyle grading system, pink diamonds are classified into three categories according to hue:
- purplish-pink (PP);
- pink (P); and
- pink rose (PR).
Each category ranges in intensity from ‘1’ (the most intense) to ‘9’ (the lightest). Stones classified as 1PP (intense purplish-pink) are generally the most expensive. The other three colour categories typically found at Argyle are: pink champagne (PC), blue-violet (BL1 ““ BL3), and red, which is the rarest colour of diamond found at Argyle.
Pink champagne (PC) colours are primarily brown diamonds with a pink secondary colour. Stones with a brown hue and pink modifier are graded using the following colour grades:
- PC1: pink champagne light;
- PC 2: pink champagne medium; and
- PC 3: pink champagne dark.
It is important Canadian jewellers be familiar with Rio Tinto’s colour grading system—all Argyle ‘better pink’ diamonds greater than .15 carats sold after 2009 are lasered with the mine’s logo and often don’t carry GIA or HRD reports.
Clarity is not as important as colour in the valuation of Argyle pink diamonds, as these stones are usually SI or I due to many inclusions, most often graphite and fractures.
Changes in certification?
Over the last five years, demand for pink diamonds has increased due to the following factors:
- “¨exposure in mainstream media, most often in the form of major celebrities buying pink diamonds, as well as record-breaking prices at major auctions;
- “¨Australia’s strict adherence to the Kimberley Process; and
- the overall rarity of pink diamonds.
As a result, pink diamonds are increasingly sought-after items at major gem and jewellery shows, with buyers often paying a premium when accompanied by paperwork proving Argyle origin.
In 2007, this author began a research project in conjunction with an international team on the characterization of pink diamonds of different origin. It found Argyle stones exhibit fluorescence under a UV lamp that corresponds to typical visible spectra and a characteristic ‘fingerprint’ in the infrared part of the spectra. While rare Golcondas are Type IIa, Argyle pinks comprise nitrogen and typically more in ‘B’ form than ‘A,’ making them Type IaAB. In collaboration with our lab, Gem Research Swisslab (GRS) in Hong Kong is also accumulating data on pink diamonds from different sources using advanced spectroscopy.
Results of this research could fundamentally change the certification process for pink and blue coloured diamonds that due to their source, share similarities with rare rubies or sapphires. Coloured stones have always been sold based on origin—consider Burmese rubies or Kashmir sapphires, for example. For diamonds, the opposite is true, although there is a possibility origin may soon be included on grading reports.