This content was first published on IGR – Italian Gemological Review no. 14 in 2022. The information provided here is therefore current as of the original publication date.
Hanmi Gemological Institute & Laboratory (HGI) of Korea reported on Journal of the Korean Crystal Growth and Crystal Technology (vol. 32, no. 1/2022) a case of moissanite coating through a polycrystalline synthetic diamond film nearly 100 times less thick than previous cases found in literature.
Five transparent stones (ranging from 0.93 to 0.96 ct, colorless, pink, yellow, blue, and red colored) have been tested. Identification as moissanite was easy as the average hydrostatic SG value was 3.21 and the samples resulted to be inert under long-wave and short-wave UV radiation. Additionally Fourier-transform infrared (FTIR) and Raman spectroscopy, identified all the specimens as synthetic moissanite.
Unlike the colored samples the colorless one did not reveal the coating trace with a standard gemological microscope, even at very high magnification (100,000x) and with most standard analytical techniques including Raman spectroscopy, energy-dispersive X-ray fluorescence (EDXRF) and scanning electron microscopy (SEM). Specifically, Raman spectroscopy investigation was unable to detect the 1332 cm-1 (produced by sp3 bonding of carbon atoms) or the ~1550 cm-1 (produced by graphite-related sp2 bonding) peak.
The Korean lab found that the coating identification of the colorless synthetic diamond coated moissanite was possible only using JEM-ARM 200F transmission electron microscopy (TEM), with an acceleration current of 200kV. The TEM image of the colorless sample revealed the presence of a 3~8 nm thick layer on the moissanite. Specifically, from the corresponding STEM Z-contrast image combined with the energy-dispersive X-ray spectroscopy (EDX) line profiles and EDX elemental maps, this layer was found to be carbon, silicon and oxygen.
Gem News published on IGR – Rivista Italiana di Gemmologia #14, Spring 2022
