Researchers at the Gemological Institute of America (GIA) recently identified a natural diamond with a CVD (lab-grown) layer, creating a composite of synthetically grown and natural diamond that added weight and improved the color. Given that this was the second discovery of such a composite, the fist coming in 2017, warning that "this could be a new type of product entering the market."
A 0.64 ct Fancy grayish-greenish-blue cushion modified brilliant showed both strong absorption of nitrogen and uncompensated boron, features characteristic of type Ia and type IIb diamonds, respectively. The lab noted, "It is very unusual for boron- and nitrogen-related defects to be seen together in natural diamonds," though it has been seen before. The lab-grown layer was greyish-blue, while the natural section had a yellowish color, giving the combined stone a fancy-greyish-greenish-blue appearance. "The resulting color was likely the main motivation for growing the CVD layer on top of the natural diamond," the lab said, "though the extra weight gained could also be a factor."
In 2017, the GIA discovered a natural white diamond covered by a thin (80 microns, or 0.003 inches) synthetic layer that colors it blue, also warning at the time that more such composites might be on the market. The 0.33-carat stone was a composite of CVD synthetic Type IIb diamond overgrowth on a natural Type Ia diamond, and the GIA noted that CVD diamond films have been applied to natural diamonds since the 1960s, while the successful addition of CVD-grown diamond layers on top of mined diamonds happened in 1993. GIA said at the time that the synthetic overgrowth on a natural diamond with a Fancy color grade is the first it has ever seen. The new case is it a thick CVD layer and not a coating of CVD overgrowth.
"With the second of these composites seen at GIA, this could be a new type of product entering the market," the GIA writes. "Earth-grown diamonds with synthetic diamond grown on the surface require extra scrutiny due to the presence of natural-looking features, both spectroscopic and gemological. Careful inspection still reveals the presence of synthetic indicators, which expose the true nature of the diamond."