As people feature their individuality (due to inherent traits, education and experience), natural diamond crystals also feature unique physical and chemical properties owing to different conditions of their primary crystallization, re-crystallization, microimpurity combination and distribution, mechanical deformations in the course of geological perturbations, as well as man-made treatment processes. Synthetic diamonds show serial properties depending on technology, apparatus and concrete growth procedure; they resemble clones, like the well-known sheep Dolly.
To identify diamonds with a 99-percent probability is the target of over a dozen of research groups, including those from Yakutniproalmaz, All-Russian Research and Development Institute of Mineral Resources named after N.M. Fedorovsky (VIMS), St. Petersburg State Mining Institute, Almazintech, a Moscow-based company, De Beers, Israel Diamond Institute, Johannesburg University.
The need for practical use of diamond dactyloscopy was discussed at the recent Kimberley Process meeting in Tel-Aviv in June 2010 to identify ‘conflict diamonds’, for example, in view of restrictions for the Zimbabwean diamonds.
Up-to-date physical and chemical techniques afford a number of opportunities
For instance there is a laser mass-spectrometry method used for diamond identification, which is being developed at the Durham University, England, and the Hebrew University, Israel, in team with the De Beers Exploration Group. The method makes possible to analyze the chemical composition of inclusions in a diamond. Its advantage is that it gives a staggeringly high precision in determination of over 20 heavy metals in a volatile diamond sample – starting from one part per trillion. Comparison between concentrations of impurity metals reveals micro-impurities in diamonds recovered from a particular deposit. Four element ratios may serve as possible identification attributes: lanthanum/ytterbium, niobium/uranium, barium/niobium, and hafnium/zirconium. At present, accumulation of measurements for diamonds is in progress involving various deposits in South Africa, Canada, Yakutia and Latin America. The weak point of this method is that a micro-sample is taken from the surface of a diamond being analyzed, so it is difficult to apply this method for non-uniform diamonds. Besides, the method is costly.
Luminescence induced by ultra-violet rays is another promising dactyloscopy technique. The luminescence spectrum is a major mineral characteristic reflecting the composition of impurities (luminogenes). Band intensities related to individual luminogenes in the spectrum may be expressed quantitatively and thus provide acceptable information for mathematical treatment, making possible to elaborate an expert system based on normalized band intensities of optically active centres in the luminescence spectrum using a probabilistic and statistic approach. By the way, comparison of characteristic bands intensity distribution in luminescence spectrum proved that the same mineral class has composition peculiarities depending on mineralization type of ore-formation it belongs to. In short, diamond groups can be referred to deposits the same way as people are related to races.
By now, systematic studies of luminescence spectra have been completed taking into consideration the attenuation kinetics of optically active centres of kimberlite and lamproite diamonds, as well as those from alluvial diamond deposits. Luminescence is formed by the whole crystal volume. Using appropriate software, mathematically treated luminescence digital image is expressed in several functions. Thanks to this, one-time-only initial applicable data obtained will ensure correct gem origin determination in future (deposit, concentrating mill, place of cutting, etc.), conformity of rough and polished diamonds, and etc.
According to Konstantin Godun, Deputy Director General, Almazintech, “the possibility to achieve a targeted result is conditioned by an experimentally established phenomenon inherent to each rough diamond and, respectively, a polished diamond received, and the phenomenon shows an extremely individual superposition of spectral and kinetic characteristics of its optically active luminescence centres. Luminescence centres are unique for every individual diamond and keep their peculiarities after diamond cutting, damage and any other exposures. The task is to elaborate technical specifications that will enable reproducibility and comparison of measurements, and the Almazintech experts have developed priority solutions of registration procedure for luminescence integrating signal, subsequent processing and comparison of measurements with database results.”
Vladimir Kudrya, Manager of Project ‘Rough and Polished Diamonds Identification’, Almazintech, said that their researchers are working to improve the identification technique using a luminescence signal, through recording it with a digital camera with subsequent mathematical treatment of the image. Photoluminescence can be generated by ultraviolet radiation of a xenon or mercury lamp, laser or light-emitting diodes.
The principle of processing a luminescence digital photo-image and visible photo-image lies in determining its pixel component distributions and mutual correlation of R, G and B components used to form pixels in digital photo-image with the help of a special algorithm. With this approach, it is possible to form a unique visualized graphic 4D (Sic!) image of a luminescence photo-image, where even a naked eye can see characteristic individual peculiarities, and use a software to introduce them into the database in a digital and graphic form. Sample identification is carried out automatically by comparison of processed measurement results with those in a database.
By the way, according to Viktor Rassulov, VIMS, this technique is applicable to zircons, substitutes of diamonds. Comparison of luminescence data in the mineral collection studied shows that samples of different ‘genetic types’ are characterized by different integral luminescence brightness and different band correlation ratio of Al-P-, Fe3+-, Dy3+-, Sm3+-centres. Zircon luminescence spectra from the Mir kimberlite pipe feature low rare earths content and maximum Al-P centre intensity.
Conclusions
Nowadays, scientific and technical knowledge open the possibility for their application, namely for:
1) reliable control over diamonds supplied for processing,
2) customs control over imported and exported gems,
3) jewelry trade,
4) criminalistics,
5) mortgage operations.
The most advanced diamond luminescence identification technique – in case an integrated database is made – will make it possible to determine average values and dispersion of these properties for individual beds, ore blocks, mines and deposits, as well as time periods for their development to solve technological tasks, and locate a mining site of each gem in the future. Besides, the proposed technique allows determining the natural or synthetic origin of crystals being analyzed, including technological enhancement of genuine gems. It also simplifies the separation process and increases its efficiency during concentration of rough stocks due to chosen optimal parameters of a distinctive feature using luminescence separation, as well as determines the equipment damaging stones (which helps preserve gems) and gives an opportunity to reliably distinguish between a rough and/or polished precious gem and a fake.
The technique is at the stage of making pilot instruments and concrete prototypes. It is expected, that in 2012-2013, the first preproduction units will be supplied to law-enforcement agenices. A relevant database is to be acquired. Commercial sales to jewelers, gemmologists and technologists are possible in 2015, at the earliest. Nevertheless, there exist more pessimistic views. For example, Maya Kopylova, Associate Professor, University of British Columbia, an expert in minerals and mineral inclusions in diamond, believes that scientific and technical progress in diamond identification will be sabotaged, for example, while creating a complete database because “a lot of major diamond corporations do not want it could be possible to determine the origin of a diamond”. However, good will of the Kimberley Process members, commitment of international law-enforcement authorities and actions of consumer rights societies are able to overcome the natural wish of business to keep the status quo in this field.
Vladimir Teslenko for Rough &Polished
