Lab Notes Gems & Gemology, Fall 2014, Vol. 50, No. 3

Diamond, Artificially Irradiated and Coated Brown

Figure 1. The diamond with a Fancy reddish brown color (left) was revealed to be Fancy Dark orangy brown once the coating and red hue were removed (right). Photos by Jian Xin (Jae) Liao.
Red is arguably the rarest and most valued color in diamond. It is known to bring record-breaking prices at auction, as in the case of the 0.95 ct Hancock Red, which sold for nearly $1 million per carat in 1987. Some will go to great lengths to achieve this color, even resorting to unusual multiple treatments.
The New York laboratory recently examined a Fancy reddish brown 0.28 ct round brilliant diamond submitted for color origin identification (figure 1). Spectroscopic analysis immediately revealed that it had been artificially irradiated to create the reddish brown color. The presence of an amber center, identified by infrared spectra, indicated that this diamond was likely brown prior to treatment. Further investigation using the DiamondView determined that the stone’s pavilion facets had also been coated, a feature not apparent with microscopic observation. DiamondView images displayed weak luminescence, with more intense luminescence concentrated at facet edges, junctions, and where the stone had been scratched. Boiling removed the coating, eradicating the red component and changing the color grade to Fancy Dark orangy brown. Removing the coating also produced a much more luminescent DiamondView image (figure 2).
Figure 2. DiamondView images of the diamond with the coating (top row) and after the coating’s removal (bottom row). The dark spots in the bottom left image are remnants of the coating substance. Images by Paul Johnson.
This stone indicates an attempt to induce red color in diamond, presumably to increase the value. Although this effort proved unsuccessful, it emphasizes the need for vigilance when analyzing diamonds with rare color. Their appeal, and the number of treatment techniques to create these colors, is sure to grow.

Martha Altobelli, Paul Johnson, and Jason Darley are researchers at GIA’s New York laboratory.