Spectral Characteristic and Colour Mechanism of Fancy Coloured CVD Diamonds—Take Blue and Pink CVD Diamonds for Example

Chemical vapor deposition (CVD) technology is widely used in the synthesis of diamond. CVD diamonds have shown significant advantages in terms of supply capacity, quality, cost and technological innovation, which has brought broad application prospects to the diamond market. The diversity of colours is an important part of the charm of diamonds, and the colour mechanism of CVD diamond always has been a hot issue. In this study, the spectral characteristics and colour mechanism of six fancy coloured CVD diamond samples (four pink samples and two blue samples) were studied using conventional gemmological tests, DiamondView^TM, FTIR, UV-Visible jewelry detector, laser Raman spectrometer and ultra-depth microscope. The following understandings were obtained: (1) Invisible inclusions in blue CVD diamond samples under 45× magnification, and there is no layered growth structure under DiamondView^TM. The pink CVD diamond samples have fine black inclusions and strong orange-red fluorescence (SW). The pink samples A01-A03 under DiamondView^TM show obvious layered growth structure, while pink sample A04 has no structural characteristics. The layered growth structure and ultraviolet fluorescence characteristics can be used as the basis for preliminary determination of CVD diamonds, as shown in Fig. 1; (2) The infrared spectra of the blue CVD diamond samples A05 and A06 have H1a centers (1 450 cm^-1), and the UV-Visible spectrum results show GR1 colour centers (741 nm). The pink CVD diamond sample A03 shows 1 330 cm^-1 solitary nitrogen absorption peak in the infrared spectrum. The pink samples A01, A02 and A04 have no relevant characterization of the existence form of nitrogen atoms in the infrared spectrum, while samples A01-A04 show [N-V]⁰ (579 nm) defect, [Si-V]^- (737 nm) defect and [Si-V]⁰ (945 nm) related absorption in the UV-Visible spectrum. Raman spectra of all CVD diamond samples show 1 335 cm^-1 diamond Raman characteristic peak, as shown in Fig. 2, Fig. 3 and Fig. 4; (3)The blue CVD diamond sample is the absorption from red light to blue light gradually weakened due to the addition of boron in the process. It has the same effect with GR1 colour center defect, H1a center caused by irradiation and annealing treatment, and the absorption band centered at 617 nm, which will lead to obvious transmission in the short wave blue light region, resulting in the blue. The colour of the pink CVD diamond sample is related to the lattice defects of nitrogen, while the samples A01-A04 have defects of [N-V]⁰ and [Si-V]^-/0, and the nitrogen impurity in sample A03 is substituted nitrogen.