In an article in GIA’s Winter 2018 edition of Gems & Gemology, authored by Vera A. Pakhomova, Dmitrii G. Fedoseev, Svetlana Y. Kultenko, Alexander A. Karabtsov, Vitaliia B. Tishkina, Valentina A. Solyanik, and Vladimir A. Kamynin, the authors have reported on the findings of a Russian lab of a synthetic piece of moissanite coated with a diamond film, in order to appear as a rough diamond.

A transparent yellowish octahedral (sawable) crystal weighing 5.35 ct was submitted as a rough diamond to the Far East Geological Institute (FEGI) for examination.

After examination, the sample was identified as synthetic moissanite that had been covered with a thin diamond film.

“The faces of the octahedron showed a stepped appearance, and the edges had parallel grooves and combinational surfaces,” the authors wrote. “The faces and edges were smooth except for one face where mechanical damage was noticeable. Initially, the specimen was examined using conventional gemmological instruments. It was inert to long-wave UV (365 nm) and anisotropic. Thermal conductivity testing indicated diamond. Tests for electrical conductivity were not convincing, however: Some areas (mostly on the edges) corresponded to diamond, but others (on smooth faces and dimples) indicated synthetic moissanite. There were no inclusions associated with natural diamond. Only small parallel needle-like inclusions, typical for synthetic moissanite, were found using a gemmological microscope.”   

They added: “X-ray tomography showed that the inner part of the sample was distinguished by its X-ray density from the outer shell, which had a variable thickness from 19 to 115 microns.  Microprobe analysis with a JEOL JXA-8100 four-channel microanalyzer revealed the presence of up to 61 wt.% silicon on some polished faces of the crystal, along with titanium in the composition of the surface layer. Some polished faces showed a diamond chemical signature.”

Based on earlier studies, the authors concluded that the diamond coating “was likely applied using polycrystalline diamonds”. The top layer, they said, was made of a composite material consisting of metal matrix and nano-diamond particles.

The authors pointed out that a diamond imitation such as synthetic moissanite, which is  coated with a thin diamond film “is challenging to identify”, since its lustre and thermal properties correspond to diamond.

“Even with a very thin coating that did not exceed 0.001 mm, the sample tested positive for diamond,” they warned. “This application of thin diamond films onto non-diamond material could be an issue for gemmologists in the future.”

Pic Cap: The 5.35 ct sample, submitted as a rough diamond, was identified as synthetic moissanite coated with a diamond film. Photo by V. Pakhomova.

Pic Courtesy: GIA