Lab Notes

Large Blue and Colourless HPHT Synthetic Diamonds


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Figure 1. The largest blue HPHT synthetic diamonds to date: a 5.26 ct heart shape and a 5.27 ct emerald cut. Both were graded as Fancy Deep blue. Photo by Johnny Leung.

The technology for producing gem-quality synthetic diamonds is making rapid progress. In May 2016, GIA’s Hong Kong laboratory examined five large HPHT synthetic diamonds grown by New Diamond Technology (NDT) in St Petersburg, Russia (table 1). Examination confirmed that all of them had the known characteristics of HPHT synthesis.

Two of the synthetic diamonds were colour graded as Fancy Deep blue (figure 1). The 5.26 ct heart shape and the 5.27 ct emerald cut both surpassed the previous record for the largest blue HPHT synthetic, a 5.02 ct specimen reported very recently. Infrared absorption spectroscopy showed that both were type IIb, with strong absorption bands from boron impurity. We observed the typical colour banding of HPHT synthetics, with bluer colour concentrated in the {111} growth sector. Photoluminescence analysis at liquid nitrogen temperature with various laser excitations revealed no impurity-related emissions, indicating that these stones were surprisingly pure in composition and lacking in defects.

Colourless and D colour HPHT synthetics
Figure 2. The emerald cut on the left is the largest colourless HPHT synthetic diamond ever reported (10.02 ct, E colour). The other two, a 5.06 ct round and a 5.05 heart, both had D colour. Photo by Johnny Leung.

The other three samples were colourless (figure 2). The largest one was a 10.02 ct emerald cut with E colour equivalent. This stone was previously reported on in 2015. The round cut weighed 5.06 ct and the heart shape 5.05 ct; both were graded as D colour equivalent. Infrared absorption spectroscopy confirmed these three were type IIa diamond, but with a very weak boron-related absorption band at ~2800 cm–1. Photoluminescence spectroscopy revealed very weak emissions from the [Si-V]doublet at 737 nm, the Nii+ doublet at 884 nm, and NV0 at 575 nm.

For all five samples, multiple growth sectors were observed in the DiamondView fluorescence images, showing features similar to other HPHT synthetic diamonds. Strong blue phosphorescence was also detected. Unlike natural type IIa or IIb diamonds, they showed no dislocation or strain when examined under a cross-polarised microscope, a strong indication of high-quality crystallisation. Their clarity ranged from VS2 to VVS2, attributed to a few tiny metallic inclusions trapped during diamond growth. No fractures were observed. All of these gemmological and spectroscopic features are consistent with typical HPHT synthetic diamonds. This material can be accurately identified with GIA’s existing protocols for analysis.

In addition to their size, these five HPHT synthetic diamonds displayed gemmological features comparable to those of top-quality natural diamonds when graded using the system for natural diamonds. This group of laboratory-grown diamonds demonstrated the quality and size that HPHT growth technology has achieved. It is obvious that more and more high-quality HPHT synthetic diamonds, including those of significant size, will be introduced to the jewellery industry. GIA’s decades of research into both HPHT and CVD synthetic diamonds allows for the ready identification of these synthetic diamonds.

TABLE 1. Large HPHT synthetic diamonds recently examined by GIA.
Sample Weight (ct) Cut Colour (ct) Clarity
1 5.26 Heart Fancy Deep blue VVS2
2 5.27 Emerald Fancy Deep blue VS1
3 10.02 Emerald E VS1
4 5.06 Round D VS2
5 5.05 Heart D VS2
 
 

Wuyi Wang is GIA’s director of research and development at GIA in New York. Terry Poon is a supervisor in the diamond identification department at GIA in Hong Kong.