Lab spots stone with CVD coating


A Chinese gemological laboratory has identified a well-concealed synthetic-diamond coating that bolstered the size of a small natural stone.

While the lower part of the stone was a colorless, natural diamond, the upper section, including the crown and the top of the pavilion, contained a layer of diamond created using chemical vapor deposition (CVD).

Although CVD coatings have been present in the industry for many years, “CVD overgrowth, which significantly influences the weight and size of the diamond, is still rarely reported up to now,” the Beijing-based National Gemstone Testing Center (NGTC), which detected the stone, said in a recent statement.

While the Gemological Institute of America (GIA) earlier this year identified an overgrowth on a natural diamond that tinted it blue, the layer that the NGTC discovered is more concerning for the trade, the Chinese group explained.

The coating over the brilliant-cut, 0.11-carat, near-colorless diamond was hard to detect, with no features betraying its existence visible using a microscope, the NGTC said. The layer it found was thicker, perhaps indicating an improvement in growth technique and, unlike in the GIA case, there were no blemishes on the stone resembling needle marks.

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Source Rapaport

Photo © Plaques diamants synthétiques CVD Courtesy of Laboratoire Français de Gemmologie.