Lab Notes Gems & Gemology, Winter 2016, Vol. 52, No. 4

Mixing of Natural Diamonds with HPHT Synthetic Melee

HPHT-grown synthetic diamond melee.
Figure 1. These 23 HPHT-grown synthetic diamond melee, ranging from 0.002 to 0.012 ct, were identified recently at GIA’s Hong Kong lab. Photo by Johnny Leung and Tony Leung.

In recent years, significant amounts of colorless to near-colorless HPHT-grown synthetic diamond melee have been produced for the jewelry industry. As a result, the separation of natural from synthetic melee diamonds has become increasingly critical. GIA offers melee diamond screening services using conventional gemological techniques and analytical methods such as photoluminescence and infrared absorption spectroscopy. In September 2016, GIA’s Hong Kong laboratory received 135 melee diamonds for identification service (see figure 1). Of these, 131 were confirmed to be HPHT synthetics and four were natural diamonds. It is interesting to find natural diamonds mixed in HPHT-dominated groups as “contamination.”

The tested melee were colorless to near-colorless round brilliants, ranging from 0.002 to 0.012 ct. Infrared absorption spectroscopy performed on the 131 HPHT synthetics showed they were generally type IIb with a very weak absorption band at ~2800 cm–1 from trace boron in the diamond lattice. Blue phosphorescence with varying intensity was observed under short-wave UV radiation (225 nm) and could be easily detected in the DiamondView (figure 2). In photoluminescence spectroscopy collected at liquid nitrogen temperature, clear emissions from SiV at 736.6/736.9 nm were recorded using 633 nm laser excitation, and extremely strong Ni-related emissions at 882/884 nm occurred in all 131 synthetic melee. These features are similar to those observed from known HPHT synthetic diamonds from a few sources in China.

Synthetic diamond melee showing phosphorescence
Figure 2. Phosphorescence in an HPHT-grown synthetic diamond melee.

The four natural diamonds showed no phosphorescence under short-wave UV radiation. When examined in the DiamondView, they displayed blue fluorescence and very weak phosphorescence. Infrared absorption spectroscopy indicated these stones were type IIa, and no trace boron absorption was recorded. In photoluminescence analysis, no SiVemission was detected. Surprisingly, all four diamonds showed weak Ni-related emissions at 882/884 nm. Their most notable photoluminescence feature was an extremely broad band centered at ~700 nm, which is usually observed in natural diamonds.

We would expect to find a small percentage of HPHT synthetics mixed in with natural diamond melee, but on a few occasions we have seen the opposite. As GIA launches the melee sorting service, we anticipate that more melee goods will be submitted for natural vs. synthetic diamond testing.

Terry Poon is supervisor of diamond identification, Carmen Lo is a quality assurance grader, and Billie Law is a diamond grader at GIA’s Hong Kong laboratory.