Lab Notes Gems & Gemology, Winter 2014, Vol. 50, No. 4

Natural Colorless Type IaB Diamond with Silicon-Vacancy Defect Center

Type IaB natural diamond
Figure 1. Emissions from the silicon-vacancy defect at 736.6 and 736.9 nm were detected in this 0.40 ct type IaB natural diamond. Photo by Johnny Leung.

The silicon-vacancy defect, or [Si-V], is one of the most important features in identifying CVD synthetic diamonds. It can be effectively detected using laser photoluminescence technology to reveal sharp doublet emissions at 736.6 and 736.9 nm. This defect is extremely rare in natural diamonds (C.M. Breeding and W. Wang, “Occurrence of the Si-V defect center in natural colorless gem diamonds,” Diamond and Related Materials, Vol. 17, No. 7–10, pp. 1335–1344) and has been detected in very few natural type IIa and IaAB diamonds over the past several years.

Recently, a 0.40 ct round brilliant diamond with D color and VS2 clarity (figure 1) was submitted to the Hong Kong laboratory for grading service. It was identified as a pure type IaB natural diamond. Infrared absorption spectroscopy showed low concentrations of the hydrogen peak (3107 cm–1) and N impurities in the B aggregates. Photoluminescence spectra at liquid-nitrogen temperature with 514 nm laser excitation revealed [Si-V] doublet emissions at 736.6 and 736.9 nm (figure 2), while 457 nm laser excitation revealed the H3 (503.2 nm) emission. DiamondView fluorescence images showed blue fluorescence with natural diamond growth patterns. These gemological and spectroscopic features confirmed the diamond’s natural origin, despite the occurrence of [Si-V] emissions. No treatment was detected.

Emission peaks in diamond PL spectrum
Figure 2. The emission peaks at 736.6 and 736.9 nm from the [Si-V] defect are shown in the type IaB diamond’s photoluminescence spectrum. Identical peak positions are detected in CVD synthetic diamonds.

Examination of this stone indicated that the [Si-V] defect can occur, albeit rarely, in multiple types of natural diamonds. Therefore, all properties should be carefully examined in reaching a conclusion when [Si-V] is present.

Carmen "Wai Kar" Lo is a diamond grader in GIA's Hong Kong laboratory.