This study experimentally confirms that β-irradiation followed by low-grade metamorphism changes the luminescence of diamonds. Three suites of Type IaA diamonds were annealed for 5 days at 500 °C, 0.4 GPa in the presence of hydrous fluid; two suites were irradiated with fast 3.5 MeV electrons prior to the annealing. Optical absorption and photoluminescence spectra of diamonds were recorded before and after the treatment. After irradiation, the diamonds acquire subtle green-blue color and GR1 (zero-phonon line at 740.9 nm) and ND1 (394 nm) optical systems, which quench after metamorphic annealing. Our experiments reproduced formation of the 574.9 and 637.2 nm centers typical for Type IaAB metamorphosed diamonds, but also created the 740.9 nm center absent in naturally metamorphosed stones. This process is explained by formation of new vacancies, their subsequent diffusion through the diamond lattice, and their entrapment at nitrogen sites, creating NVN, NV0, and NV− centers. Diamonds affected by β-irradiation should be more common in the upper crust than α-irradiated diamonds, and we provide examples of naturally occurring diamonds of blue-green color that may have formed due to β-irradiation.
A surprising result of our study is a significant decrease in optical absorption at 350–700 nm and a disappearance of light brown color in diamonds after the relatively low-temperature metamorphic annealing. This transparency enhancement may have resulted from deep cleaning of the diamond surface and near-surface microfractures by the supercritical hydrous fluid present in the experimental charge.