Lab Notes Gems & Gemology, Spring 2017, Vol. 53, No. 1

HPHT Synthetic Diamond Melee Without Si or Ni Defects

Near-colorless HPHT synthetic melee.
Figure 1. These four near-colorless round brilliant melee were identified as HPHT synthetic diamonds lacking the typical Si- and Ni-related defects. Photo by Nuttapol Kitdee.

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.

PL spectra of HPHT synthetic melee.
Figure 2. The PL spectra features observed at liquid nitrogen temperature lacked the 736.6/736.9 nm emission doublet related to the SiV defect (left, 514 laser excitation) and the Ni-related defect doublet at 883/884 nm (right, 830 nm laser excitation).

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.

DiamondView images show growth patterns of HPHT synthetics.
Figure 3. These DiamondView fluorescence and phosphorescence images of a 0.081 ct sample show growth patterns characteristic of HPHT synthetic diamond. Image by Charuwong Khowpong.
Darkfield illumination reveals rod-like inclusions.
Figure 4. Darkfield illumination reveals rod-like inclusions in a 0.087 ct HPHT-grown melee diamond. Photomicrograph by Charuwan Khowpong and Ungkhana Atikarnsakul.

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.

Piradee Siritheerakul is a gemological manager at GIA’s lab in Bangkok. Wuyi Wang is GIA’s director of research and development.