Gem News International Gems & Gemology, Fall 2013, Vol. 49, No. 3

Unusual Dumbbell-Like Inclusion in Diamond

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Figure 4. This 0.40 ct round brilliant diamond displayed a large, unusually shaped inclusion. Photo by Aurélien Delaunay.
The French Gemmological Laboratory (LFG) in Paris had the opportunity to grade a 0.40 ct near-colorless round brilliant diamond with a large, unusual inclusion (figure 4). Initial inspection under the loupe showed an unusual internal feature consisting of a thin middle “bar” and two broader appendices (figure 5). Viewed under certain angles, it appeared almost metallic. This feature, which lowered the clarity grade to SI2, was reminiscent of the metallic dumbbell inclusions observed in HPHT synthetic diamonds. 

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Figure 5. At first inspection, this dumbbell shape was reminiscent of the metallic flux inclusions seen in HPHT synthetic diamond. Photomicrograph by Aurélien Delaunay; magnified 120×.
DiamondView luminescence images demonstrated that the inclusion was crystallographically oriented, parallel to the edge between two octahedral faces (figure 6). Dumbbell inclusions in HPHT synthetics have a clear crystallographic orientation, often along an octahedral edge as well. 

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Figure 6. DiamondView luminescence images demonstrated the diamond’s natural origin, as only traces of octahedral growth were found. The inclusion is crystallographically oriented, parallel to the edge between two octahedral faces, as highlighted by the slightly more inert elongated rectangle beside it. Photomicrograph by Aurélien Delaunay, magnified approximately 60×.
But the stone fluoresced weak blue under long-wave UV radiation and was inert in shortwave UV, a classic luminescence behavior for natural diamond and unknown in synthetic specimens. Furthermore, the infrared absorption spectrum was typical of a natural type IaAB diamond with minor hydrogen. This proved the natural origin of both the stone and its inclusion. With magnification, the inclusion appeared colorless and transparent rather than metallic and opaque, though with a high optical relief. Its morphology was not cuboctahedral—as with inclusions in synthetic diamond—but rounded octahedral. The exact nature of the crystal remains unknown, as we could not obtain a Raman spectrum due to its intense luminescence, but colorless inclusions in diamonds are often forsterite.

Emmanuel Fritsch is a researcher at the Institut des Matériaux Jean Rouxel (IMN), University of Nantes, France; and Aurélien Delaunay is a researcher at the French Gemmological Laboratory in Paris.