The Gemological Institute of America (GIA) has discovered a synthetic layer that improved the weight and color of a natural diamond, and has warned that the phenomenon may be happening more often.
The cushion modified brilliant, 0.64-carat stone contained about 0.10 carats of chemical vapor deposition (CVD) diamond, the GIA estimated in a lab note last week. The lab-grown layer was greyish-blue, in contrast to the natural section’s yellowish color, giving the combined stone a fancy-greyish-greenish-blue appearance.
This was not the first time a stone of this type has turned up at the GIA: In 2017, it reported on a 0.33-carat, fancy-blue diamond that featured a CVD overgrowth similar to this one.
“With the second of these composites seen at GIA, this could be a new type of product entering the market,” research associate Troy Ardon and analytics technician Garrett McElhenny wrote in the note, which the GIA published in the Spring 2019 issue of its quarterly scientific journal, Gems & Gemology. “The resulting color was likely the main motivation for growing the CVD layer on top of the natural diamond, though the extra weight gained could also be a factor.”
The stone’s unusual nature came to light after testing indicated it had features of both type Ia and type IIb diamonds — a rare combination. It appeared to have absorbed both nitrogen — a feature of type Ia diamonds that gives yellow color — and boron, which is present in type IIb stones and can turn them blue. “Mixed-type diamonds always call for additional scrutiny,” Ardon and McElhenny explained.
The pavilion — the section from the girdle to the bottom — showed natural growth features during fluorescence testing with DiamondView, a De Beers machine for identifying synthetics. However, the crown displayed characteristics of CVD, proving that manufacturers had grown CVD diamond over a natural base. Using computer modeling, the research team was able to calculate the weight of the synthetic part as approximately one-tenth of a carat.
The stone was well disguised: Numerous readings of its photoluminescence showed no indication of any synthetic origin, despite the fact that such tests are usually effective at revealing CVD. This may be because the lab-grown layer was so thin, the note pointed out.
“Natural diamonds with synthetic diamond grown on the surface require extra scrutiny due to the presence of natural-looking features, both spectroscopic and gemological,” the authors continued. “Careful inspection still reveals the presence of synthetic indicators, which expose the true nature of the diamond.”