Gem News International Gems & Gemology, Spring 2018, Vol. 54, No. 1

Irradiated and Annealed Blue Type Ia Diamond

Wenqing Huang, Xuguang Jin, Wei Cai, and Guanghai Shi

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This fancy deep green-blue diamond was treated in a multi-step process.
Figure 1. This 0.36 ct fancy deep green-blue diamond is a multi-step treated diamond. Photo by Wenqing Huang.

Many diamonds are subjected to artificial radiation, usually with subsequent annealing to create attractive colors. Most blue diamonds in the market are treated by artificial irradiation, yet irradiated blue diamonds with a multi-step treatment process are rarely seen in the lab. The National Center of Supervision and Inspection on Quality of Gold and Silver Products (Nanjing, China) recently examined such an example.

DiamondView of treated blue diamond.
Figure 2. The DiamondView image showed a characteristic ring-like natural growth pattern. Image by Wenqing Huang.

The 0.36 ct round brilliant, graded as fancy deep green-blue (figure 1), had natural mineral inclusions under the table and a fracture near the girdle. The DiamondView image showed a ring-like natural growth pattern (figure 2). The infrared spectrum revealed the diamond was type IaAB, with absorptions at 1282 cm–1 and 1175 cm–1 (A-aggregated nitrogen and B-aggregated nitrogen, respectively); the spectrum also revealed a radiation-related peak at 1450 cm–1 (figure 3). The H1a center reflected by the 1450 cm–1 band, involving a single nitrogen atom and two equivalent carbon atoms, was formed by irradiation and annealing above 300°C for diamonds with aggregated nitrogen (type Ia) (I. Kiflawi et al., “Nitrogen interstitials in diamond,” Physical Review B, Vol. 54, No. 23, 1996, pp. 16719–16726). The UV-Vis-NIR absorption spectrum showed absorption from a GR1 defect at 741.2 nm and revealed weak absorptions from a defect of uncertain structure (594.4 nm), H3 (503.2 nm), and H4 (496.2 nm) centers (figure 4). The GR1 center, believed to be a neutral vacancy (V0) defect, is a typical radiation-induced defect; the vacancies begin to move at 500°C and the GR1 center begins to decrease until it disappears, at around 800°C (A.T. Collins, “Investigating artificially coloured diamonds,” Nature, Vol. 273, No. 5664, 1978, pp. 654–655). The 595 nm center is also a typical radiation-induced center, which appears at temperatures above 300°C after radiation (Collins et al., 1978). The presence of the H3 and H4 centers was confirmed with photoluminescence (PL) spectroscopy (figure 5). The combination of all the above defects, especially the strong H1a center, indicates that the diamond was subjected to irradiation followed by annealing.

FTIR spectrum of treated blue diamond.
 
Figure 3. The FTIR spectrum of the 0.36 ct irradiated diamond revealed the sample was type IaAB, with absorptions at 1282 cm–1 (A-aggregated nitrogen) and 1175 cm–1 (B-aggregated nitrogen). The H1a absorption at 1450 cm–1, associated with irradiation and subsequent heating, was also detected. In addition, the spectra showed peaks at 1405, 1096, and 1010 cm–1, all of which are usually seen in type Ia diamond.
UV-Vis-NIR spectrum of treated blue diamond.
 
Figure 4. The UV-Vis-NIR spectrum of the 0.36 ct irradiated diamond, collected at liquid nitrogen temperature, showed a clear GR1 center defect at 741.2 nm, and weak absorption lines at 594.4 nm, 503.2 nm, and 496.2 nm.
PL spectrum of treated blue diamond.
 
Figure 5. The PL spectrum of the sample excited with a 405 nm laser showed weak emissions at 496.2 nm (the H4 center) and 503.2 nm (the H3 center). A peak at 491.7 nm was also detected.

Natural near-colorless type Ia diamonds are known to be artificially irradiated to introduce blue to green color by displacing carbon atoms in the lattice to create vacancies. Annealing could be used to change the lattice defect configuration, especially GR1, to achieve a purer blue (C.M. Breeding, “A spectroscopic look at green and blue gem diamonds colored by artificial irradiation treatment,” presentation at Geological Society of American 2014 annual meeting). After annealing above 500°C, the vacancies start to migrate, and can be captured by nitrogen centers to create the H1a center (B. Dischler, Handbook of Spectral Lines in Diamond, Volume 1: Tables and Interpretations, Springer-Verlag, Berlin, 2012, pp. 315–316). To the best of our knowledge, this is the first diamond examined in China that has been irradiated and annealed to introduce a blue color.

Wenqing Huang, Xuguang Jin, and Wei Cai are affiliated with the National Center of Supervision and Inspection on Quality of Gold and Silver Products Nanjing, China. Guanghai Shi is affiliated with the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing.