Color Enhancement of Natural Type Ia Diamond Using a Neutron–Irradiation and Annealing Method

Abstract

Color in diamonds arises from defects or impurities such as nitrogen, boron or a lattice vacancy. Therefore, suitable types and quantities of defects will assist in creating the most desirable color in a diamond. In this research, ESR spectroscopy was used to find the optimal annealing temperature for color changing in a neutron-irradiated type Ia diamond. It was found that negative vacancies (V-) became mobile above 600°C and were completely trapped by nitrogen to form color centers at 750°C. Therefore, the optimal temperature for color treatment of irradiated diamonds was 750°C. After annealing, yellow diamond was obtained. The optimal fluence of neutron irradiation to produce green diamond was then determined. This research showed that neutron irradiation could turn brown diamond into green diamond with the shading dependent on the fluence of irradiation. The optimal fluence to obtain green diamond was 1.58×10¹⁶ n/cm², while at higher levels the diamond became opaque. After annealing at approximately 750°C, irradiated diamond turned into yellow diamond. A higher fluence of neutron irradiation provided a relatively dark yellow color after annealing. The pale or dark shade of enhanced yellow diamond seemed to be independent of the annealing time.