The East Coast laboratory is occasionally asked to examine diamond rough prior to cutting. Recently, we received two diamond crystals, both with requests to determine whether their color was natural or the result of treatment. One crystal was approximately half a carat, and the second was about one carat. Both had worn edges and generally frosty surfaces with small, evenly distributed, crescent-shaped fractures and a few larger indentations. In reflected light, the smaller crystal appeared to be blue-green (see figure), while the larger crystal was blue.

Our suspicions were first raised when the smaller stone was viewed in transmitted light and a pale yellow bodycolor was evident. On close inspection of the crystal surface, we noted a black to dark green flaky material in the largest indentation, as well as green concentrations in the shallow crescent-shaped cracks.

When this crystal was examined with a desk-model spectroscope, it exhibited classic cape series absorption features (a line at 415 nm due to the N3 center, along with an N2 peak at 478 nm). As a result, we concluded that this particular piece of diamond rough was naturally yellow but had been surface dyed to produce an apparent blue-green color.

The larger crystal also exhibited different colors in reflected and transmitted light: It appeared blue in reflected light, but greenish gray in transmitted light. Indentations in the crystal surface contained a dark blue flaky material, shallow surface fractures exhibited concentrations of blue color, and lighter blue spots were seen on the surface of the rough. When examined with a desk-model spectroscope, the larger crystal exhibited only a weak absorption feature at 415 nm. These observations were consistent with the larger crystal being gray to near colorless with a blue surface dye. Undoubtedly, in both cases, the dyed surface would have been removed during faceting.

Many gem materials are routinely dyed to alter or improve their color appearance, but dyed diamond rough is unusual. In fact, this is the first report of such rough to appear in Gems & Gemology. These specimens are another reminder that simple “traditional” treatment methods, such as dyeing and coating (see, e.g., D. J. F. Evans et al., “Coated pink diamond—A cautionary tale,” Spring 2005 Gems & Gemology, pp. 36–41), should not be forgotten when determining the origin of a diamond’s color, whether polished or rough.

Carolyn van der Bogert
GIA Laboratory, New York