Lab Notes Gems & Gemology, Fall 2021, Vol. 57, No. 3

New CVD Material Submitted for Analysis

These fairly large CVD-grown diamonds have high color and clarity grades.
Figure 1. These 10 CVD-grown diamonds submitted to the New York laboratory were fairly large (2.24 ct to 5.90 ct) with high color grades (D to F) and excellent clarity (VVS2 to VS1). Photo by Sood Oil (Judy) Chia.

Since the introduction of its new digital Laboratory-Grown Diamond service, GIA has recently seen a vast increase in the number of diamonds grown by chemical vapor deposition (CVD). These are often large and of exceptional clarity. The New York laboratory recently examined a batch of 10 large, high-quality CVD-grown diamonds from a single client, Shanghai Zhengshi Technology. The diamonds ranged from 2.24 ct to 5.90 ct and came in a variety of fancy shapes, as well as the standard round brilliant cut. All of them had high color grades, ranging from D to F, and excellent clarity, with grades between VVS2 and VS1 (figure 1).

PL spectrum of an as-grown CVD diamond.
Figure 2. PL spectrum showing the 596/597 nm doublet observed in an as-grown CVD diamond.

The diamonds were all identified as type IIa using Fourier-transform infrared (FTIR) spectroscopy, with the 3123 cm–1 peak that is attributed to most as-grown CVD synthetics and the 3017 cm–1 peak that can be found in treated CVD synthetics both notably absent from their spectra. PL spectroscopy using a 514 nm laser revealed SiV (737 nm), NV (637 nm), and NV0 (575 nm) centers in all of the diamonds. Also observed was the 596/597 nm doublet, a feature commonly seen in as-grown CVD diamond indicating no treatment was applied (figure 2) (S. Odake, “Melee diamonds: Metal defects and treated color,” Fall 2018 G&G, p. 304).

Strong birefringence pattern shows many interference colors.
Figure 3. Each CVD-grown diamond showed a strong birefringence pattern displaying many interference colors. The pattern is not disrupted by the presence of pinpoint inclusions in the diamonds. Photomicrographs by A’Dhi Lall and Elina Myagkaya. Field of view 19.27 mm (left) and 1.26 mm (right).
Homogenous, unbroken dislocations of CVD-grown diamonds.
Figure 4. DiamondView imaging revealed homogenous, unbroken dislocations without the distinct banding that is common in CVD-grown diamonds. Images by Elina Myagkaya.

The diamonds showed strong birefringence when viewed under cross-polarized light, exhibiting both low and high interference colors (figure 3). DiamondView imaging revealed mostly blue, purple, or pink fluorescence, with blue-violet dislocations that were clearly observed in all of the diamonds tested. More notable was the absence of the striations commonly seen in CVD diamonds, which indicate start-stop growth and changing growth conditions (figure 4). The absence of striations implies that these diamonds might have been grown in one continuous step—the fact that the dislocations appear to be homogenous and uninterrupted supports this theory.

This group of lab-grown diamonds possessed high clarity and high color for as-grown material, demonstrating the potential for large lab-grown diamonds to make large inroads in the gem diamond market. With continuing improvements to growth technology, lab-grown diamond identification faces many challenges. While large batches of CVD synthetics from different manufacturers have been documented by GIA in the past (e.g., W. Wang et al., “CVD synthetic diamonds from Gemesis Corp.,” Summer 2012 G&G, pp. 80–97), this set offers insight into potential new CVD growth conditions for CVD synthetic diamonds.

Elina Myagkaya is a research associate, and Paul Johnson is manager of analytics, at GIA in New York.