Lab Notes Gems & Gemology, Spring 2015, Vol. 51, No. 1

Analysis of Melee Diamonds Using FTIR Spectroscopy

Colorless round brilliant melee
Figure 1. This 0.00054 ct colorless round brilliant was submitted for identification as a natural or synthetic diamond. Despite the specimen’s small size, an FTIR microscope can focus a beam through the facets to capture a high-quality infrared spectrum. Photo by Jian Xin (Jae) Liao.

The jewelry industry has expressed increasing concern over the possibility of treated or synthetic diamonds being mixed in with natural melee goods (those weighing less than 0.20 ct). Recently, GIA’s laboratory has seen a surge in notably small faceted melee diamonds submitted for identification. In melee sizes less than 0.01 ct, a standard Fourier-transform infrared (FTIR) spectrometer provides a less stable environment for analysis and produces an indistinct spectrum. Diamonds smaller than 0.01 ct prove challenging for other screening devices as well. Meanwhile, there is heightened pressure to identify both loose and mounted melee diamonds in the laboratory quickly and reliably so that each stone can be analyzed. In late 2014, GIA developed a new protocol to use an FTIR microscope to focus on melee diamonds as small as 0.00054 ct (figure 1) to produce high-quality spectra suitable for diamond typing.

Operating the FTIR microscope in reflection mode allows for fine-tuned aligning of the beam within a sample, improving the detail of the infrared spectra. Once a faceted round brilliant melee is stationed on a slide, the microscope beam can be focused either through the pavilion (if the melee is table-down) or through the table at an angle to reflect off an inner pavilion facet (if the melee is resting on the pavilion). Many of the samples were so small that the position of the stone remained unknown until the microscope was focused. Nevertheless, spectral quality was independent of stone position.

All 70 melee examined were natural round brilliants, the smallest a 0.00054 ct colorless sample identified as a type Ia diamond (figure 2). A few screening devices for treatment and synthetics have been introduced for diamonds over 0.01 ct. The FTIR microscope has proved very effective in analyzing melee smaller than this size, either loose or mounted.

Melee brilliant spectrum
Figure 2. This infrared spectrum of the 0.00054 ct diamond in figure 4, obtained using an FTIR microscope, shows the detail that can be captured from very small melee sizes.

The knowledge that GIA can type such small stones and determine their origin, either loose or mounted, apart from being an impressive test of FTIR microscopy, will increase consumer confidence in these remarkable melee diamonds.

Rachel Sheppard is a research technician, Tom Moses is executive vice president and chief laboratory and research officer, and Wuyi Wang is director of research and development at GIA in New York.