Color is a defining characteristic of gem diamonds, influenced significantly by the presence of H3 (2N+V) and H4 (4N+2V) color centers. These defects impact both absorption and fluorescence, contributing to the attractive fancy yellow-green hues observed in some diamonds. H3 and H4 centers can form naturally in the Earth’s crust through long-term geological processes involving irradiation and annealing. However, they can also be artificially produced by exposing diamonds to high-energy electron beams followed by laboratory annealing. Distinguishing between naturally and artificially formed color centers is critically important for the gemological industry.

In this study, we report the occurrence of H3 and H4 centers in diamonds from a unique alluvial deposit in Africa. These diamonds, averaging approximately 3 carats in size, were found in quartz pebble and cobble conglomerates overlying the basal sediments of the Umkondo Group (1.11 Ga). The surfaces of the diamonds exhibited extensive green and brown radiation stains, indicating strong natural irradiation and annealing. As a result, significant concentrations of H3 and H4 centers were introduced, and some diamonds with appropriate nitrogen levels displayed fancy yellow-green coloration. This marks the first confirmed discovery of a diamond source consistently producing such naturally colored diamonds.

To compare with artificial processes, 17 colorless diamonds containing suitable nitrogen concentrations were irradiated using a 2 MeV electron beam and subsequently annealed at high temperatures. These treated diamonds also developed fancy yellow-green colors due to the formation of H3 and H4 centers.

Both natural and treated diamonds were extensively analyzed using a range of spectroscopic techniques, including infrared and UV-Visible absorption spectroscopy, and photoluminescence spectroscopy at liquid nitrogen temperatures using laser excitations at 830 nm, 633 nm, 532 nm, 514 nm, and 457 nm. The study identified distinct spectroscopic differences between natural and artificially treated diamonds, enabling reliable separation in gemological testing. While the formation mechanisms of H3 and H4 centers are fundamentally similar, differences in irradiation intensity, exposure duration, and annealing conditions—particularly the formation of additional lattice defects such as interstitials—play a key role in distinguishing the two origins.