Results from spectroscopic analyses of GE POL high-pressure high-temperature (HPHT) annealed nominally type IIa diamonds are presented, and these spectral characteristics are compared with those of untreated diamonds of similar appearance and type. Absorption spectroscopy reveals that any yellow coloration in such HPHT-treated diamonds is due to low concentrations of single nitrogen, which have not been observed in untreated type IIa diamonds. Laser-excited photoluminescence spectroscopy reveals the presence of nitrogen-vacancy centers in most, but not all, HPHT-treated stones. When these centers are present, the ratio of the 575:637 nm luminescence intensities offers a potential means of separating HPHT-treated from untreated type IIa diamonds. The total absence of luminescence may be another important indicator of HPHT treatment.
