IRRADIATED AND ANNEALED NATURAL PINK DIAMONDS

Natural irradiation and annealing can produce green and blue colors in diamond, e.g. 41 carats Dresden Green diamond. Artificial irradiation and annealing can also create attractive colors in gem-quality diamonds. In gemology, it has always been a challenge to identify correctly between naturally and artificially irradiated diamonds.

We irradiated and annealed 20 natural diamonds in this study. Diamonds are type Ib (contains single substitutional nitrogen atoms, C center) and mixed type diamonds (bearing pair of nitrogen atoms (A center), nitrogen aggregates (B center) and C center). Electron beam (1MeV) was applied for 4 hours, and annealed the samples at 1000 °C for one hour. The colors of diamonds were gray-yellow to brown-yellow before treatment. After irradiation and annealing, they turned to attractive pink colored diamonds.

The treatment introduced radiation-related peak at 1450 cm^-1 (H1a center) as revealed in the IR spectrum. A sharp peak at 637 nm and its phonon side bands can be observed in the absorption spectrum. This peak is attributed to intense pink color. During irradiation process, vacancies are formed. They become mobile and trap at single nitrogen atoms to form nitrogen-vacancy (NV) centers during annealing. The negatively charged NV centers are responsible for 637 nm peak. A sharp H2 peak (negatively charged nitrogen-vacancy-nitrogen defect) was introduced in the mixed type diamonds. The DiamondView™ images, which are excited by <250 nm short-wave UV show that dislocation lines were destroyed by the treatment. The irradiated diamonds are color graded at GIA as Fancy Deep/Fancy Dark brown-pink to purple-pink color. Spectroscopic analyses and DiamondView™ imaging are useful to detect artificial irradiation and annealing in natural diamonds.