Black Diamonds Colored by Hydrogen Clouds
Paul Johnson of the GIA Laboratory in New York submitted the following entry to the Gems & Gemology (G&G) Lab Notes section.
The vast majority of black diamonds submitted to the Laboratory have been treated by high-temperature heating in a vacuum. These stones are usually opaque and heavily fractured. Black graphite inclusions, induced by heating, block light transmission and cause the host diamond to appear black.
Recently at the GIA Lab in New York, we examined two cut-cornered rectangular brilliants, weighing 2.69 ct and 2.72 ct, that were submitted at the same time. Both were color graded Fancy black (figure 1), but unlike treated black diamonds, these two stones were transparent to fiber-optic illumination and contained numerous dense hydrogen clouds but no evidence of graphite inclusions. A strong 3107 cm-1 peak, attributed to hydrogen, was visible in their mid-infrared absorption spectra. After nitrogen, hydrogen was the most common impurity in these diamonds and was responsible for the natural black color.
The two diamonds were almost identical, and we suspected that they had originated from the same crystal. This was supported by DiamondView images of the two stones, which showed striking almost-mirror-image butterfly structures in the center of their table facets (figure 2).
The distribution of these symmetrical hydrogen clouds throughout the crystal was concentrated in the cubic growth sectors of the diamond lattice. The tables of both diamonds were cut and oriented parallel to the cubic crystal face direction, resulting in the butterfly images that indicated their origin from the same piece of rough.
The grading reports issued for these two stones stated that they had a natural origin of color.
Figure 1. The black color of these diamonds