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Diamond has long been cherished for its unique beauty and hardness, and researchers have tried to create diamond in their laboratories for centuries. Nowadays, all artificial diamonds in our trade are created by either one of two currently known methods. The first, high pressure high temperature (HPHT) synthesis which uses a press to recreate the conditions at which diamonds grow in nature. The second, Chemical Vapour Deposition (CVD), is a method to artificially grow diamonds in a factory in a vacuum chamber and at moderate temperatures.

Since its humble beginnings in the 1950s, the production of CVD synthetics has progressed substantially. While growing gem-quality diamond by CVD was once considered exceptionally difficult, polished stones of over 5 carats have been reported (e.g. Eaton-Magaña, S. and Shigley J.E., Observations on CVD-Grown Synthetic Diamonds: A Review. Gems & Gemology, Fall 2016, Vol. 52, No. 3). Thus, being able to identify such samples unambiguously has become increasingly important to ensure disclosure at all stages of the diamond trade. Red fluorescence under short-wave UV light, such as that provided by DiamondView is one distinguishing feature of CVD synthetic diamonds (see figure 1).

As-grown CVD synthetics are brownish and require post-treatment to generate a more attractive colour. High pressure high temperature treatment can be used to reduce or remove the brown hue completely, leaving the stone colourless. This colour change is due to changes in the crystallographic defects within the synthetic diamond during HPHT treatment. Crystallographic defects are tiny imperfections and impurities in the regular arrangement of atoms in a gemstone. The defects themselves are invisible to the naked eye but can cause colour. Their “fingerprint” can be detected by spectroscopy in a gemmological laboratory and aids in distinguishing natural diamond of natural colour from synthetic diamond, either treated or untreated, including, of course, CVD synthetic diamonds.

Photoluminescence (PL) spectroscopy is one of the tools used for the authentication of diamonds. This technique uses a laser to excite these defects and allows a gemmologist to determine whether the stone is natural. Some of the most important features of the PL “fingerprint” of CVD synthetic diamonds are the doublets (pairs of peaks) occurring at 737 nm, 563 nm and 596 nm. However, some of these are occasionally detected in natural diamonds as well as CVD synthetics and some disappear or decrease during HPHT treatment (see figure 2) making them difficult to detect. Thus, SSEF has recently acquired a blue laser to complement the setup. When using this new laser, two lines at 467 nm and 482 nm are clearly visible, results which have only been reported in CVD diamonds.

Thus, photoluminescence excited by the new blue (405 nm) laser adds one more piece to the puzzle of authentication methods used at SSEF. These methods include photoluminescence excited not only with our new blue laser (405 nm) but also with another green laser (514 nm), IR spectroscopy and UV-Vis-NIR spectroscopy to name just a few. The complexity of processing and interpreting an ever-growing range of data sets has inspired us to expand and fine-tune our current automatized spectral processing.

Figure 1: DiamondView image of a CVD synthetic diamond slab showing characteristic red fluorescence. The sample was acquired by SSEF for research purposes and later HPHT treated.

Figure 2: Photoluminescence spectrum of the CVD synthetic diamond shown above before and after HPHT treatment excited with a green laser (514 nm, spectrum collected at low temperature, spectra shifted for clarity).
Figure 3: Photoluminescence spectrum of the CVD synthetic diamond shown above after HPHT treatment excited with a blue laser (405 nm). The 482 nm Si-related and 467 nm lines, features unique to CVD synthetics, are clearly visible.

Laura Speich is a Gemologist in the diamond department at the Swiss Gemmological Institute SSEF. She holds a Bachelor and Master of Science degrees in Geosciences from Ruhr-University Bochum (Germany). During her PhD in geology at the University of Bristol (United Kingdom), which she completed in 2017, she focused on platelet defects in diamond and their application as a mantle thermometer.

Jean-Pierre Chalain is head of the diamond department & deputy director at the Swiss Gemmological Institute SSEF. He completed his Diploma of Gemmology at the University of Nantes in 1992. His main working fields in the laboratory are colourless and coloured diamonds, but he is also involved with the testing of coloured stones and pearls. Jean-Pierre is a delegate of Switzerland at the World Jewellery Confederation (CIBJO).

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