Lab Notes Gems & Gemology, Spring 2013, Vol. 49, No. 1

Very Large CVD-Grown Synthetic Diamond

Very Large CVD-Grown Synthetic Diamond
Figure 1. This 2.16 ct marquise (13.42 × 6.73 × 3.94 mm) is the largest CVD synthetic diamond tested by GIA. It had J/K color, with a brownish tint.
In the last decade, we have witnessed rapid improvement in the quality and size of lab-grown CVD synthetic diamonds. The New York laboratory recently tested a 2.16 ct CVD synthetic from Scio Diamond Technology Corp. (figure 1), the largest specimen GIA has seen so far.
This marquise, which measured 13.42 × 6.73 × 3.94 mm, was more than twice the size of a 1.05 ct pear shape examined in 2010, previously the largest CVD synthetic tested by GIA (see Summer 2010 G&G Lab Notes, pp. 143–144). It had J/K color with a brownish tint, and microscopic observation revealed small fractures, pinpoints, and non-diamond-carbon black inclusions, resulting in an SI2 clarity grade.

The mid-infrared absorption spectrum revealed that the diamond could be classified as type IIa. It was very pure in the infrared region, with no nitrogen  or hydrogen-related absorption features. Photoluminescence analysis at liquid-nitrogen temperature with varying laser excitations displayed typical CVD features: very strong emissions from N-V centers, moderate emission lines at 736.6/736.9 nm from the [Si-V]– center, and the CVD-specific 596/597 nm pair (figure 20). Also detected was weak emission from the H3 defect with a zero-phonon line at 503.1 nm, making for an interesting combination of optical centers. Under the short-wave UV radiation of the DiamondView, the table and crown facets showed pure blue fluorescence while the pavilion displayed banded orange fluorescence with irregular blue fluorescence regions. Very strong internal stress with irregular patterns was confirmed under the microscope using cross-polarized light.

Testing of this large, gem-quality CVD synthetic underscored the rapid improvement of lab-grown diamond technology. It is foreseeable that more of these products will continue to reach the jewelry industry.