Lab Notes Gems & Gemology, Summer 2015, Vol. 51, No. 2

Near-Colorless Melee-Sized HPHT Synthetic Diamonds Identified in GIA Laboratory

Six near-colorless HPHT synthetic melee diamonds
Figure 1. These six near-colorless round brilliant melee, ranging from 0.01 to 0.005 ct, proved to be HPHT synthetic diamonds. Photo by Nuttapol Kitdee.
With significant development in diamond treatments and synthesis over the last decade, the trade has serious concerns about treated and/or synthetic material mixed with natural melee-sized stones. The mixing of treated and synthetic diamonds with yellow natural melees was previously reported (Winter 2014 Lab Notes, pp. 293–294; Spring 2015 Lab Notes, pp. 64–65). In order to ensure consumer confidence, it is essential to screen melees so as to distinguish all treated and synthetic goods.

Recently, the Bangkok lab had the opportunity to examine six melee-sized specimens submitted for synthetics and treatment screening. The transparent, near-colorless round brilliants ranged in size from 0.005 to 0.01 ct (figure 1). Microscopic observation revealed no obvious inclusions. Between crossed polarizing filters, very little or no strain was observed. Infrared absorption spectra showed that four of the samples were type IIa diamonds with no detectable defect-related absorption, while the other two were type IIb diamonds accompanied by uncompensated boron showing moderately strong absorption at approximately 2800 cm–1. Photoluminescence spectra obtained at liquid-nitrogen temperature with 514 and 633 nm laser excitations revealed an emission doublet of the negatively charged silicon split-vacancy defect [Si-V]at 736.6/736.9 nm for five of the samples, while 830 nm laser excitation showed an emission doublet at 883.0/884.7 nm owing to a nickel-related defect for all six samples (figure 2). DiamondView imaging revealed either blue or green fluorescence, together with characteristic growth features of HPHT synthesis, as seen in figure 3. Strong blue phosphorescence was also observed for all six samples. These gemological and spectroscopic features confirmed that the specimens were grown by HPHT synthesis.

PL Spectra of near-colorless HPHT synthetic melee diamond
Figure 2. Photoluminescence spectra at liquid-nitrogen temperature of a small near-colorless HPHT synthetic diamond. The spectra displayed emission peaks at 736.6 and 736.9 nm related to a [Si-V]– defect (left, 514 nm laser excitation) and at 883.0 and 884.7 nm associated with a Ni-related defect (right, 830 nm laser excitation).
DiamondView image of one of the HPHT synthetic melee diamonds
Figure 3. This DiamondView image of one sample (third from the left in figure 1) shows the characteristic growth features of HPHT-grown synthetic diamonds. DiamondView image by Charuwan Khowpong.
To the best of our knowledge, this is the first examination of near-colorless HPHT synthetics of these sizes by a gemological laboratory. Discovery of these HPHT synthetic diamond melee reaffirms the need to screen the huge volume of near-colorless melee currently in the trade. 

Wasura Soonthorntantikul is a staff gemologist, and Piradee Siritheerakul is gemological manager, at GIA’s Bangkok laboratory.