This study of type IIa GE POL diamonds before and after HPHT annealing by GE significantly expands on their characterization. The color change was dramatic: from the N–O range through Fancy Light brown before, to D–H after (see figure). However, there was little change to the inclusions, graining, and strain as a result of HPHT exposure. Photoluminescence (PL) studies—conducted at liquid helium, liquid nitrogen, and room temperatures in the 245–700 nm range—identified a significant reconfiguration of the lattice involving substitutional impurities, vacancies, and interstitials. Key regions of PL activity included the areas of the N3, H3, and N-V centers. X-ray topography identified the extent of lattice distortion. Cathodoluminescence may help establish that a diamond is not HPHT annealed. A distinction between non-enhanced and color-enhanced type IIa diamonds can be made through a combination of observations and features.

(Caption)Ten of the study samples before HPHT processing. These type IIa diamonds received color grades from the N-O range to Fancy Light Brown (C3-C5 on the Argyle scale). Following processing by GE, the color improved dramatically, to D to H. Photo by Phillip Hitz.