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

Pseudo-Synthetic Growth Structure Observed in Natural Diamond

Lab Notes
Figure 1. This 0.70 ct, D-color natural round brilliant displayed the cuboctahedral growth structure of an HPHT-grown synthetic diamond. Photo by Jian Xin(Jae) Liao.

Synthetic diamonds created using the traditional HPHT (high-pressure, high-temperature) process ordinarily grow as cuboctahedra. Temperature dictates the crystal form, as cubic growth is predominant at the relatively low temperatures of synthesis. At the higher temperatures of natural formation environments, diamonds typically grow as octahedrons. While small cuboctahedral diamonds are found in nature, these are very rare.

Recently submitted to the GIA laboratory in Israel was a near-colorless 0.70 ct round brilliant (figure 1). Found to be a type IIa diamond with no detectable nitrogen impurity, it was examined further for possible treatments and to verify the origin of color.

Figure 2. DiamondView imaging of the pavilion facets revealed a subtle growth structure. The inset shows the growth structure of a type IIb synthetic diamond. Photo by Wuyi Wang.

In the short-wave UV radiation of the DiamondView, a subtle cuboctahedral growth structure (figure 2) was observed on the pavilion facets. This type of structure usually indicates an HPHT-grown synthetic diamond.

The diamond possessed D color and high clarity, with no internal inclusions to help indicate whether it was in fact natural. Shallow surface-reaching fractures were observed, and a few extra facets close to the pavilion contained these natural-looking fractures.

Microscopic observation with cross-polarized light showed relatively strong tatami strain pattern (figure 3), a feature indicative of natural growth. Further examination at higher magnification revealed small polygonal dislocation networks on the pavilion. These provided conclusive evidence that the stone was a natural diamond crystal (figure 4).

Lab Notes
Figure 3. A tatami strain pattern, observed under crossed-polarized light, indicated natural growth. Magnified 40×. Photo by Paul Johnson.
Lab Notes
Figure 4. DiamondView imaging showed a micro-dislocation network on pavilion facets, conclusive proof of natural crystal growth. Photo by Wuyi Wang.

This stone was a good example of a very rare natural diamond exhibiting synthetic growth characteristics. It exemplifies the challenges posed to gemological laboratories in separating natural from undisclosed gem-quality synthetic diamonds in today’s jewelry market. We concluded that the diamond had a natural color origin.