The Gemological Institute of America (GIA) has reported detecting a natural 0.33-carat Type Ia white diamond with a synthetic, Type IIb synthetic layer applied on top to colour it a fancy blue. The thickness of the CVD overgrowth layer was approximately 80 microns.
Nitrogen is the most abundant defect in natural diamonds, while boron, on the other hand, is a rare impurity. The GIA New York laboratory was alerted by the fact that it saw both nitrogen and boron defects in a single diamond.
CVD synthetic diamond films have been grown on natural diamond substrates since the early 1960s and in 1993, CVD synthetic layers were successfully grown on natural type IIa and Ia diamond substrates. However, the GIA says this is the first time it has seen a synthetic overgrowth on a natural diamond with a fancy colour grade.
The GIA cautioned that identification of coloured diamonds should be performed very carefully by looking for unusual characteristics, such as a straight boundary line associated with an interface plane, and fluorescence zones with sharp edges in DiamondView images. Examination of this fancy-coloured composite diamond indicated that similar challenges could exist for colourless and near-colourless diamonds.