Lab Notes Gems & Gemology, Summer 2017, Vol. 53, No. 2

HPHT Synthetic Diamond Melee Found In Mounted Jewelry

One melee in this ring is an HPHT synthetic diamond.
Figure 1. Among the 70 melee diamonds mounted in this ring, one was confirmed to be an HPHT synthetic diamond. Photo by Johnny Leung.

Separation of treated and synthetic diamond melee from natural diamond is challenging work. Diamond melee, which weigh less than 0.2 ct, are often submitted to gemological laboratories in parcels containing hundreds (or more) loose stones. GIA’s automatic screening device, introduced in 2016, can separate synthetic and treated diamond melee from natural diamond melee. Identification becomes more challenging when the melee are mounted, due to their small size and the difficulty of accessing them.

In late February 2017, GIA’s Hong Kong laboratory received a ring containing 70 melee in addition to a marquise-cut diamond center stone (figure 1). Using a prototype of GIA’s new screening device for mounted gems, 69 of the melee passed the test for natural diamond, and one was referred as possible treated or synthetic diamond. The stone in question, which was set next to the center marquise diamond (figure 2, left), had no apparent inclusions. Infrared absorption spectroscopy identified it as a type IIb diamond with an absorption band at 2800 cm–1, indicating the presence of boron as a defect, forming a local vibration mode. Using photoluminescence (PL) spectroscopy, a very strong doublet peak at 882/884 nm related to nickel impurity was recorded under laser excitation and low temperature. The NV center was more prominent than the NV0 center. Strong blue phosphorescence was also detected (figure 2). These observations confirmed that this was an HPHT synthetic diamond, very similar to melee we have previously tested that were manufactured in China.

Fluorescence and phosphorescence in mounted synthetic melee diamond.
Figure 2. Set near the center marquise diamond, one melee diamond (left, in red circle) showed the blue fluorescence (center) and phosphorescence (right) characteristic of HPHT synthetic diamond. Photos by Johnny Leung (left) and Terry Poon (center and right).

How to screen jewelry items with mounted diamonds for quality control is a concern in the industry. GIA’s robust instrument for rapid and accurate screening of small diamonds set in mountings will be available in late 2017.

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

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