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Low-quality polycrystalline natural diamonds become black when annealing them in an inert atmosphere at sufficiently high temperatures. Different carbon phases (the coexistence of diamond, graphite and amorphous carbon) are identified after this treatment using Raman spectroscopy. Internal structural changes in the microstructure upon annealing are reported and defects are characterised by transmission electron microscopy (TEM) analysis. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Willems et al. (2004) studied this question.
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