Near-colorless synthetic diamond melee grown using the high-pressure, high-temperature (HPHT) method have created significant concern in the industry, and various screening methods have been developed to take into account their most reliable distinguishing features. In general, the specimens described in previous Lab Notes (e.g. Summer 2015, pp. 183–184) were very small round faceted goods ranging between 0.005 and 0.01 ct.
Recently GIA’s Bangkok laboratory received four loose near-colorless round brilliants for quality assurance service (figure 1). They weighed between 0.07 and 0.09 ct, slightly more than previously submitted melee. All were identified as type IIb HPHT-grown material using Fourier-transform infrared (FTIR) spectroscopy. Their most interesting feature was the absence of silicon- or nickel-related emission peaks when analyzed at liquid nitrogen temperature by photoluminescence (PL) spectroscopy (figure 2). The spectra typically observed in HPHT synthetic diamonds show an emission doublet related to the negatively charged silicon split-vacancy defect SiVˉ at 736.6/736.9 nm and an associated Ni-related defect at 883.0/884.7 nm.
The growth patterns observed in the DiamondView were an identifying feature that did remain the same. The characteristic angular growth patterns and blue-green fluorescence, together with an associated green phosphorescence, still provide important evidence that aid in the separation of natural and synthetic diamonds (figure 3). Examination with a gemological microscope revealed metallic and rod-like inclusions (figure 4) that also assisted in identifying these melee as synthetic diamonds.
The study of these four melee-size HPHT synthetic diamonds revealed atypical PL spectra. However, other means of identification (inclusions, growth patterns as seen via the DiamondView, and their phosphorescence reactions) still played an important role in their identification. It is interesting to note that HPHT synthetic melee diamonds being submitted to GIA for identification are getting larger, indicating the continuous improvement of synthetic technology.