FTIR spectrometer has the advantage of performing in situ measurements of HDO absorbance peak at 2504 cm⁻¹ with high sensitivity of 0.01‰.

Paramagnetic centers in diamonds are affected by nitrogen atom contamination in the lattice. This study aimed to better understand the nitrogen concentration and anisotropy impact on the EPR (electron paramagnetic resonance) spectrum of natural diamonds (i.e. appropriate for color enhancement treatments). All the diamonds used were natural either colorless or faintly colored (D–M color range), which were selected according to the targeted final color post-treatment: green, blue, pink, orange, and yellow. Their paramagnetic centers were studied using EPR spectroscopy at various microwave power levels and evaluated for nitrogen concentration via FTIR spectroscopy. The results show a correlation between nitrogen concentrations and spin concentrations in all the pretreated diamonds, whereas the most abundant fast-relaxing paramagnetic center is similar to the C-centered radical. In addition, the hyperfine interaction anisotropy of the paramagnetic centers of nearby nitrogen atoms was found to be valid for pretreated green, pink, and orange diamonds. By clarifying the final structure of the treated fancy diamonds, which are visibly affected by adjacent nitrogen atoms, this study contributes to the refinement of colorless natural diamond selection for color enhancement.

The modification of nitrogen-contaminated diamonds into color-enhanced diamonds is usually achieved by irradiation and thermal treatment (annealing). These treatments affect nitrogen contamination chemical bonding, vacancy concentration, and atom orientation centers in the diamond lattice. In this study, natural diamonds were subjected to irradiation and thermal annealing color enhancement treatments to produce green, blue, and yellow fancy diamonds. The study followed the changes that occur during treatment relying on visual assessment, fluorescence, UV-vis, FTIR, and EPR spectroscopy to characterize paramagnetic centers. The results indicated that diamonds containing high levels of nitrogen contamination presented a relatively high carbon-centered radical concentration. Two paramagnetic groups with different g-values were found, namely, low g-value centers of 2.0017–2.0027 and high g-value centers of 2.0028–2.0035. It is suggested that the 2.0017–2.0022 centers correlate with blue centers, whereas the 2.0023–2.0027 centers correlate with yellow centers. It was also found that thermal treatment was required to produce blue and yellow fancy diamonds, whereas no such treatment was needed to produce green diamonds.

Color induction in nitrogen-contaminated diamonds was carried out via various procedures that involve irradiation, thermal treatments (annealing), and more. These treatments affect vacancy defect production and atom orientation centers in the diamond lattice. Natural diamonds underwent color enhancement treatments in order to produce green, blue, and yellow fancy diamonds. The aim of this study was to follow the changes occurring during the treatment, mainly by EPR spectroscopy, which is the main source for the determination of the effect of paramagnetic centers (carbon-centered radicals) on the color centers produced via the treatments, but also via visual assessment, fluorescence, UV-vis, and FTIR spectroscopy. The results indicate that diamonds containing high levels of nitrogen contamination are associated with high carbon-centered radical concentrations. Four paramagnetic center structures (N1, N4, and P2/W21) were generated by the treatment. It is suggested that the N4 structure correlates with the formation of blue color centers, whereas yellow color centers are attributed to the presence of N1 species. While to produce blue and yellow colors, a thermal treatment is needed after irradiation, for treated green diamonds, no thermal treatment is needed (only irradiation).

Diamonds contain carbon paramagnetic centers (stable carbon radicals) in small concentrations (at the level of ~1 × 10¹² spins/mg) that can help in elucidating the structure of the nitrogen atoms’ contaminants in the diamond crystal. All diamonds that undergo polishing are exposed to high temperatures, owing to the friction force during the polishing process, which may affect the carbon-centered radicals’ concentration and structure. The temperature is increased appreciably; consequently, the black body radiation in the visible range turns orange. During polishing, diamonds emit an orange light (at a wavelength of about 600 nm) that is typical of a black body temperature of 900 °C or higher. Other processes in which color-enhanced diamonds are exposed to high temperatures are thermal treatments or the high-pressure, high-temperature (HPHT) process in which the brown color (resulting from plastic deformation) is bleached. The aim of the study was to examine how thermal treatment and polishing influence the paramagnetic centers in the diamond. For this purpose, four rough diamonds were studied: two underwent a polishing process, and the other two were thermally treated at 650 °C and 1000 °C. The diamonds were analyzed pre- and post-treatment by EPR (Electron Paramagnetic resonance), FTIR (Fourier transform infrared, fluorescence, and their visual appearance. The results indicate that the polishing process results in much more than just thermal heating the paramagnetic centers.