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An Investigation Into the Cause of Color in Natural Black Diamonds from Siberia

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An Investigation Into the Cause of Color in Natural Black Diamonds from Siberia NOTES & NEW TECHNIQUES AN INVESTIGATION INTO THE CAUSE OF COLOR IN NATURAL BLACK DIAMONDS FROM SIBERIA Sergey V. Titkov, Nikolay G. Zudin, Anatoliy I. Gorshkov, Anatoliy V. Sivtsov, and Larisa O. Magazina with an optical microscope have been described Black and dark gray diamonds from Siberia, previously as graphite (see, e.g., Kammerling et al., Russia, were studied by analytical scanning and 1990). Orlov (1977) suggested that finely dispersed transmission electron microscopy. Their color is graphite microparticles also occur in those black caused by the presence of dark inclusions. Unlike diamonds in which individual inclusions could some previous reports on black diamonds in not be seen with the optical microscope. which the dark inclusions were primarily graphite, Although very rare, facetable black diamonds the Siberian samples with the most intense black are found in primary kimberlite and secondary color contained predominantly magnetite inclu- alluvial deposits worldwide. Such diamonds are sions, while the dark gray diamonds most com- mined in Siberia, eastern Russia, mainly from the monly contained inclusions of hematite and native Mir kimberlite pipe. A number of companies, such iron. Moreover, the black diamonds studied exhib- as Rony Carob Ltd. (Moscow), V. N. Almaz LLC ited anomalously high magnetic susceptibility, which may serve as one criterion for determining (New Jersey), and V. B. International Corp. (New the natural origin of black color. York), cut black diamonds from Siberia for use in jewelry (see, e.g., figures 2 and 3). The aim of this article is to document the iden- atural diamonds occur in almost all colors tification of mineral inclusions in black and near- (Orlov, 1977; Hofer, 1998). Most colors— black diamonds from Siberian deposits using ana- Nincluding brown, pink, red, orange, yellow, lytical scanning and transmission electron green, blue, and violet—are caused by defects in the microscopy methods. In our previous work diamond’s crystal structure, most commonly nitro- (Gorshkov et al., 2000; Titkov et al., 2001; and ref- gen and boron impurities, vacancies, and disloca- erences therein), we used these techniques to tions (Collins, 1982; Fritsch and Rossman, 1988). In investigate microinclusions in polycrystalline dia- contrast, black and gray colors in untreated natural mond aggregates (bort and carbonado) from various diamonds are due to the presence of mineral inclu- deposits. That data expanded our knowledge of sions rather than structural defects. inclusions in natural diamonds, and demonstrated Strange as it may seem, the nature of these their mineralogical diversity. In particular, we inclusions has not been studied in detail, although found that native metals—including common Fe high-quality faceted black diamonds have attracted and rare Cr, Ni, Ag, Au, Cu, Ti, and Fe-Cr, Fe-Ni, considerable interest in the marketplace (see, e.g., Gruosi, 1999; figure 1). Data on mineral inclusions in black diamonds available in the gemological lit- See end of article for About the Authors and Acknowledgments. erature are based mainly on examination with the GEMS & GEMOLOGY, Vol. 39, No. 3, pp. 200–209. optical microscope. Black inclusions clearly seen © 2003 Gemological Institute of America 200 NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY FALL 2003 Figure 1. Black dia- monds have become increasingly popular in modern jewelry such as this brooch, which con- tains 992 black dia- monds (total weight 54.50 ct) and 170 color- less diamonds (total weight 1.90 ct). The cen- ter diamond weighs 2.51 ct. Courtesy of Chanel Fine Jewelry, from the Collection Privée. Ag-Au alloys—occur widely as inclusions in natu- formed using a JEOL JEM-100C transmission elec- ral polycrystalline diamonds. It is noteworthy that tron microscope (TEM) equipped with a goniome- some of these metals, most often Ni and Fe, are ter and a Kevex 5100 energy-dispersive X-ray spec- used as fluxes in diamond synthesis, and synthetic trometer, allowing the detection of elements from crystals often contain them as inclusions Na to U. We also used a JEOL JSM-5300 scanning (Muncke, 1979). electron microscope (SEM) equipped with an Oxford LINK ISIS energy-dispersive X-ray spec- trometer, which permitted detection of elements MATERIALS AND METHODS from Be to U (including, most significantly, oxy- We studied four rough black diamonds (samples gen). With these two instruments, we obtained 1–3 and 6), and two rough nearly black or dark gray information on micromorphological features (from diamonds (samples 4 and 5), from which polished SEM backscattered electron images and TEM brilliants could be manufactured. Samples 1–5 images), line and volume structural defects (from were recovered from the kimberlites of western TEM images), structural parameters (from select- Yakutia and weighed 13–25 ct each; sample 6, 1.8 ed-area electron diffraction patterns obtained using ct, came from the Anabar placer deposit in north- the TEM), and chemical compositions (from ener- ern Yakutia. gy-dispersive spectra) of inclusions as small as a All six diamonds were examined with an fraction of a micron. Box A provides more informa- Olympus BX51 optical microscope, and photomi- tion on analytical electron microscopy. crographs were taken using a MIN-8 microscope In preparation for analysis, each sample was with a Pentax-MZ5N camera. crushed after being wrapped in a special thick The six diamonds were then studied with ana- paper to avoid contamination. In large fragments, lytical electron microscopy (Wenk et al., 1995; quantitative energy-dispersive analysis of inclu- Shindo and Oikawa, 2002), and the inclusions sions was performed by SEM on polished surfaces, were identified by their chemical composition and and semi-quantitative analysis was performed on structural parameters. Investigations were per- rough, unpolished surfaces that were quite flat and NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY FALL 2003 201 BOX A: ANALYTICAL ELECTRON MICROSCOPY Analytical electron microscopy is the combination of backscattered electrons reflected from a sample sur- transmission or scanning electron microscopy (TEM face. SEM provides information on micromorpholo- or SEM) with energy-dispersive X-ray spectroscopy gy, grain and pore sizes, sample homogeneity, the (EDS). It is one of the most powerful tools available relationship between different phases, and the like. for characterizing the solid-state substances that For both TEM and SEM, the interaction of the occur as micron-sized inclusions in minerals. electron beam with a sample produces characteris- The electron microscope uses a beam of acceler- tic X-ray radiation. Measuring the intensities of ated electrons instead of visible light (as in an optical these X-rays with energy-dispersive detectors (the microscope). Electron microscopy provides for mag- basis of EDS) reveals the chemical composition of nifications of more than 500,000×, and a spatial reso- the sample. lution of 3 angstroms (or 3 × 10−10 m), because the In summary, analytical electron microscopy wavelength of an electron beam is about four orders provides unique information on micromorphologi- of magnitude shorter than that of visible light. cal features (from SEM images and TEM images), In TEM, the electron beam is focused by a mag- line and volume structural defects (from TEM netic lens through a specimen that is thin enough images), structural parameters (from electron to be transparent to the beam. In image mode, it diffraction patterns obtained using the TEM), and provides electron micrographs that reveal various chemical composition (from EDS) of samples as defects (dislocations, misoriented microblocks, small as a fraction of a micron. For more informa- microtwins, etc.) in a specimen. In diffraction tion on analytical electron microscopy, see Wenk mode, the instrument generates selected-area elec- et al. (1995), Shindo and Oikawa (2002), and refer- tron diffraction patterns, calculation of which ences therein. reveals structural characteristics. The general prin- Facilities equipped with SEM+EDS and ciple behind electron diffraction patterns is similar TEM+EDS instruments can be found at some uni- to that of X-ray diffraction patterns. Analysis of versities or research centers, particularly those spe- these patterns is very complicated, because pub- cializing in materials science, chemistry, physics, or lished lattice parameters obtained with this tech- geology. The approximate costs of these instru- nique are quite rare in the mineralogical literature, ments are typically: SEM, $100,000–$300,000; and absent from the gemological literature. The TEM, $500,000–$1,000,000; and EDS detector, analysis and calculation of electron diffraction pat- $70,000–$300,000. terns are discussed in Hirsch et al. (1977). Electron diffraction patterns may be of two types: point and ring. Point diffraction patterns are formed if the sample studied is a perfect single crys- tal, with each point corresponding to the reflection from a specific set of atomic planes in the lattice. Ring diffraction patterns are produced by polycrys- talline aggregates, with each ring corresponding to a certain reflection from many microblocks, or grains, in various orientations (i.e., a ring consists of many points, each of which corresponds to one misorient- ed block). Discrete-ring diffraction patterns may arise if microblocks in the aggregate have a pre- ferred orientation, or if the aggregate is character- ized by a certain texture. Some important limitations of the TEM tech- nique in gemology are that the sample must be very thin (i.e., it must be crushed to a powder or chemi- cally etched), and the area analyzed is only about 1 Figure A-1. The scanning electron microscope used mm in diameter. for this study was equipped with an energy-disper- In a scanning electron microscope (see, e.g., figure sive X-ray spectrometer, for analysis of elements A-1), images are recorded using secondary and ranging from Be to U. Photo by M. K. Sukhanov. 202 NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY FALL 2003 Figure 2. These two faceted diamonds are from the Siberian deposits in Russia, which were also the source of the samples examined for this study.
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