Two yellow kite-shaped step cuts (1.77 and 1.70 ct.; see figure 1) were submitted to GIA’s West Coast laboratory by Mitchell Swerdlow Inc. of Miami, for diamond color origin determination. Initial observation in the laboratory weights and measures department indicated specific gravity values that were close to 6 for both, requiring additional testing by the Identification staff.
Subsequent observation of the spectra seen with the desk-model spectroscope appeared to reveal a cape spectrum in both samples, with the addition of a line at 595 nanometers (nm), which is often indicative of irradiation and annealing in a yellow diamond. However, due to the high density of the specimens, it was obvious that they were in fact not diamonds at all, and further testing was clearly required.
No inclusions were seen in either sample when examined with magnification, and there was no visible strain. Lack of doubling in the microscope suggested that the kite shapes were singly refractive, and they were over the limits of the refractometer. In addition, they fluoresced weak orangy red to long-wave ultraviolet (UV) radiation and medium to strong orangy red to short-wave UV, which is also not consistent with yellow diamonds.
When the visible absorption spectra were examined more closely, it became apparent that although they did closely resemble the spectrum of a treated cape diamond, the 415 nm band was missing, and the positions of some of the main bands were shifted slightly on the scale. The spectra of cape diamonds contain bands in the violet-to-blue region, with lines at approximately 415, 435, 452, 465, and 478 nm; the 415 and 478 nm bands are the strongest. When a diamond is artificially irradiated and annealed to produce a yellow color, the cape lines are usually fairly weak, and there are often additional lines at 496 and 503 nm and a weak line at about 594 nm. Figure 2 compares the spectrum of the yellow samples we received for identification with a representative spectrum from a cape diamond. Due to the close proximity of lines on the scale, at first glance the 485 nm band in the samples could have been confused with the comparatively strong 478 nm band in a cape diamond, although the two samples also had an even closer but weaker line around 473 nm. The samples also had a fine weak line around 594 nm, which could have been confused with the same line in an irradiated and annealed diamond, but the more prominent band at 585 nm in the samples is really the culprit in a potentially mistaken comparison.
To further characterize the spectra of the two samples, research scientist Andy H. Shen collected UV-visible spectra. The absorption features presented themselves generally as small broad bands, and for the most part they corresponded to the lines observed in the desk-model spectroscope. However, the weak feature at about 594 nm was not visible.
Energy-dispersive X-ray fluorescence (EDXRF) analysis performed by senior research associate Sam Muhlmeister revealed zirconium as the major chemical component, along with a trace of yttrium and hafnium. These findings were consistent with cubic zirconia. As this note illustrates, if one is using a desk-model spectroscope to quickly confirm the identification of a “yellow diamond,” one must be aware that the spectrum of a yellow CZ can be mistaken for that of a cape or treated cape diamond. The lines should be studied closely to avoid a potentially embarrassing error.
