Two Natural Type IIa Diamonds with Strong Phosphorescence and Ni-Related Defects
Strong phosphorescence under UV excitation is rarely seen in natural diamond and normally limited to hydrogen-rich type Ia or type IaA/Ib chameleons and type IIb diamonds (T. Hainschwang et al., “A gemological study of a collection of chameleon diamonds,” Spring 2005 G&G, pp. 20–35; S. Eaton-Magaña and R. Lu, “Phosphorescence in type IIb diamonds,” Diamond and Related Materials, Vol. 20, No. 7, 2011, pp. 983–989). When seen in other diamond types, an even rarer occurrence, it is shorter and less intense. Recently, the National Gemstone Testing Center (NGTC) in Beijing encountered two natural diamonds that showed extraordinarily strong blue phosphorescence and uncommon fluorescence colors under the DiamondView.
The diamonds weighed 0.18 and 0.30 ct and were both graded as E color. Both were type IIa, with neither nitrogen absorption between 1100 and 1400 cm–1 nor boron-related absorption in their infrared spectra. Magnification and examination with a standard UV lamp showed no abnormal phenomena for natural type IIa diamonds. Instead of the deep blue fluorescence and inert phosphorescence that natural type IIa diamonds typically show in DiamondView imaging, the 0.18 ct diamond emitted unevenly distributed grayish blue fluorescence and strong blue phosphorescence, similar to that of a colorless HPHT synthetic diamond. The 0.30 ct diamond displayed vivid red fluorescence and strong blue phosphorescence, which is unusual for a natural colorless to near-colorless diamond (figure 1). Red fluorescence is rare in natural type IIa diamond and seldom reported (Summer 2016 Lab Notes, pp. 189–190).
Photoluminescence (PL) spectra collected with 532 nm laser excitation at liquid nitrogen temperature revealed something even more interesting. Along with the GR1 center emission at 741 nm typically seen in natural type IIa diamonds, the 883.0/884.7 nm doublet that is related to nickel impurity appeared in both samples’ PL spectra (figure 2). This doublet, often referred to as the “1.40 eV center,” is frequently seen in the {111} growth sectors of HPHT synthetic diamonds produced using Ni-based solvents/catalysts. Ni-related defects are more often seen in natural chameleon and greenish yellow diamonds (W. Wang et al., “Natural type Ia diamond with green-yellow color due to Ni-related defects,” Fall 2007 G&G, pp. 240–243; Summer 2014 Lab Notes, pp. 151–152) but seldom found in colorless to near-colorless natural diamonds (e.g., Spring 2017 Lab Notes, pp. 95–96). For natural diamonds with such strong phosphorescence, the presence of Ni-related emissions is unusual.
The PL spectrum of the 0.30 ct sample also presented an extremely strong NV center at 575 nm, compared to the diamond emission at 572 nm, which explained the reddish fluorescence in the DiamondView image. In each sample’s PL spectrum, the relative intensity of NV0 (575 nm) is significantly stronger than that of NV– (637 nm), another sign of natural origin.
Furthermore, when we exposed the 0.30 ct sample to higher-energy excitation with the 532 nm laser to reveal more subtle features, the PL spectrum showed moderately strong emissions at 648/649 nm and 776 nm (figure 3). According to previous research, PL peaks at 648.2 and 776.4 nm are associated with boron in phosphorescing type IIb natural diamonds (Eaton-Magaña and Lu, 2011). Later studies ascribed the 648.2 nm defect to a boron-interstitial complex, while the 776.4 nm peak was assigned to a B-V complex (S. Eaton-Magaña and T. Ardon, “Temperature effects on luminescence centers in natural type IIb diamonds,” Diamond and Related Materials, Vol. 69, 2016, pp. 86–95). These emissions may partly explain the strong phosphorescence of the 0.30 ct sample. Since they were not detected in the 0.18 ct sample, which also exhibited strong phosphorescence—the same is true of other phosphorescent natural IIa diamonds the authors have encountered before—this cannot be the full explanation.
Despite the synthetic-like luminescence color in DiamondView imaging and the Ni-related emissions in the PL spectra, the samples’ Fourier-transform infrared (FTIR) spectra, UV-visible spectra, PL spectra, features observed under the crossed polarizing microscope, and UV reaction indicated that they are natural instead of synthetic. These two samples demonstrated the diversity of luminescence and spectral features found in natural diamond. Their identification also highlights the importance of a comprehensive understanding of diamond origin.
