Large Blue and Colorless HPHT Synthetic Diamonds

The technology for producing gem-quality synthetic diamonds is making rapid progress. In May 2016, 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 color 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 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 color banding of HPHT synthetics, with more blue color concentrated in the {111} growth sector. Photoluminescence analysis at liquid nitrogen temperature with various laser excitations revealed no impurity-related emissions, indicating these stones were surprisingly pure in composition and lacking in defects.

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

The other three samples were colorless (figure 2). The largest one was a 10.02 ct emerald cut with E color equivalent. This stone was previously reported in 2015. The round cut weighed 5.06 ct and the heart shape 5.05 ct; both were graded as D color 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 Ni_i⁺ doublet at 884 nm, and NV⁰ at 575 nm.

For all five samples, multiple growth sectors were observed in 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-polarized microscope, a strong indication of high-quality crystallization. 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 gemological and spectroscopic features are consistent with typical HPHT synthetic diamonds. This material can be accurately identified with existing protocols for analysis.

In addition to their size, these five HPHT synthetic diamonds displayed gemological 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 HPHT growth technology has achieved. It is obvious that more and more high-quality HPHT synthetic diamonds, including those with significant size, will be introduced to the jewelry industry.