Distinguishing Characteristics of the Lucent Imperial Diamonds

Etched surfaces: Areas around the girdle/pavilion that had not been repolished after treatment showed evidence of severe etching. This is a clear indicator of HPHT treatment, said Smith. “Etch features were also evident in surface-reaching cleavages or fractures,” he noted, adding that etching creates a coarse or granular appearance on the surface of the stones and along fracture walls.

Graphitized coating on mineral inclusions: “When diamonds are taken to these temperatures and pressures, it is common for the surfaces of the diamond (both external and internal-surrounding included minerals and along fractures) to partially convert to graphite. “The texture, color, and appearance of these inclusions are very characteristic,” said Smith. In addition, the conversion to graphite requires about 30 percent more space, so internally, the resulting pressure it exerts may cause additional stress fractures around included crystals or as extensions of existing fractures,” said Smith.

Color zoning: This is especially useful for identification, said Smith. “Color concentrations are one of the most distinctive characteristics of this treatment.” As he showed several illustrations, Smith said, “Purple and pink-to-red color concentrations often occurred with brown concentrations and regions that were near-colorless. This type of color zoning is not consistent with naturally colored diamonds." He also noted that no color concentrations were observed at the culet.

Strain patterns. Smith said the most commonly banded strain patterns were observed, showing a low degree of internal strain. Some showed a higher degree of strain, with a combination of banded and mottled patterns, he said, while others revealed a high degree of internal strain.

Fluorescence: Another very diagnostic feature for the identification of these treated color diamonds was their characteristic fluorescence reaction to both long-wave and short-wave UV light. The Lucent diamonds exhibited chalky fluorescence to long-wave UV, usually in combinations of yellow, green, and orange. Under short-wave UV, they showed medium to strong orange fluorescence, commonly with weak yellow zones, Smith said.

This image reveals the typical, chalky long-wave UV fluorescence of the “Imperial” treated-color diamonds, which results from a combination of orange, yellow, and green fluorescing zones. Such a reaction is not seen in natural-color pink-to-red and purple diamonds. Photomicrograph by C. P. Smith; magnified 10x.

The reaction to short-wave UV of these diamonds is also highly indicative of the treatment, displaying a consistent moderate-to-strong orange and yellow fluorescence, shown in this image. In most samples, the orange fluorescence dominated; the yellow was seen only in certain areas, such as near the culet in this sample. Photomicrograph by C. P. Smith; magnified 10x.

Visible Luminescence: Smith said prominent green visible luminescence was typical, and it resulted from high concentrations of H3 centers (an absorption feature at 503 nm). Orange-red visible luminescence was also seen, and was caused by the nitrogen-vacancy defect (N-V)^–center positioned at 637 nanometers (nm). Yellow visible luminescence was due to the (N-V)⁰ center positioned at 575 nm.

Spectroscopy: In its research, GIA used several spectrometers to identify the occurrence and concentration of various defect centers to identify the presence of the treatment. These included Raman photoluminescence (PL), Infrared, and UV/Vis/NIR spectroscopy. Within the infrared region, it was shown that the diamonds were originally type Ia, containing nitrogen in various states of aggregation. The HPHT steps of the treatment process facilitate the de-aggregation of nitrogen defects, resulting in single substitutional nitrogen (Ib component), as well as other defects, while the subsequent high-energy electron irradiation and relatively lower temperature annealing provide a further reconfiguration of defects to produce the necessary color-causing centers, in addition to others that are characteristic of these treatment methods.                               

Low temperature Raman PL and UV/Vis/NIR spectroscopy further revealed a combination of strong absorptions of N-V centers at 637 and 575 nm, 594, H3/H4 at 503, and 496 nm, N3 at 415 nm, as well as others, the combination of which Smith indicated have never been found in natural-color diamonds.