Gem-quality synthetic diamond grown by chemical vapor deposition (CVD) is rarely encountered among the stones submitted for a
GIA diamond report. Rarer still are type IIb CVD specimens—most are type IIa. Until recently, only one type IIb CVD synthetic had been submitted (Summer 2008 Lab Notes, pp. 158–159).
In April 2012, the Carlsbad laboratory examined a 0.25 ct round brilliant with Faint brown (L) color and I1 clarity. The sample showed strong graining with magnification, and it fluoresced very weak yellow to long-wave UV radiation and weak orange to short-wave UV. No phosphorescence or electrical conductivity was observed. Its DiamondView reaction was marked by moderately strong orangy red, mottled purple, and two unusual concentric rings of blue fluorescence.
DiamondView imaging also showed moderate blue phosphorescence, with the areas of strongest phosphorescence corresponding to the blue-fluorescing rings. The growth conditions leading to this unusual pattern are unknown. Nevertheless, similar luminescence colors and mottled purple areas have been seen previously in CVD synthetics.
The infrared absorption spectrum revealed no detectable nitrogen, but did show weak absorption at 2800 cm-1 from trace amounts of boron. The Vis-NIR spectrum showed a gradual increase in absorption toward shorter wavelengths, which imparted the brown color. Conversely, type IIb diamonds usually show a gradual increase in absorption toward longer wavelengths, resulting in a blue color.
Additionally, this synthetic diamond produced strong absorption from the Si-V defect (doublet at 736.6/736.9 nm), which was also manifested as a very strong photoluminescence (PL) emission. Si-V is a common, often diagnostic defect in CVD synthetic diamond.
Based on the sample's spectroscopic and gemological features, GIA readily identified it as a type IIb CVD synthetic diamond. The specimen was unusual due to its boron content and distinctive DiamondView characteristics.
Image: This 0.25 ct round brilliant with L color and I1 clarity proved to be a type IIb CVD synthetic diamond (top). Its DiamondView fluorescence image (bottom) showed regions of orangy red and blue fluorescence, including two distinctive blue concentric rings. Photos by Robison McMurtry (top) and S. Eaton-Magaña (bottom).
Courtesy of GIA (Gemological Institute of America) © 2012.