Lab Notes Gems & Gemology, Spring 2013, Vol. 49, No. 1

Yellow Synthetic Sapphire Colored by Trapped-Hole Mechanism

SP13 LN Fig.21
Figure 1. The UV-visible spectrum of this yellow sapphire shows broadband absorption features below ~500 nm, originating from trapped holes associated with Mg and Cr. The DiamondView image clearly displays curved bands (sub-parallel to the long direction of the stone under the table), indicating a synthetic origin. These curved bands are not seen with standard immersion, demonstrating the DiamondView’s effectiveness (the red color is due to the fluorescence of chromium). The stone’s CIE L*a*b* color coordinates are reproduced from the measured UV-visible spectrum.
Natural sapphires with pale or lemon yellow color and no orange hue are traditionally associated with the chromophore Fe3+, which substitutes for aluminum in corundum. The New York laboratory recently received a 2.58 ct yellow emerald-cut stone (figure 1, inset), identified as sapphire by its RI and SG. But the desk-model spectroscope showed broadband absorption below ~500 nm without any of the Fe3+-related absorption features associated with yellow coloration (e.g., at 450 nm). Microscopic observation showed an exceptionally clean interior with only a small cluster of tiny particles that resembled gas bubbles from synthetic materials. The stone fluoresced medium orange to long-wave UV radiation and inert to short-wave UV.
The suspicious inclusion scene and the absence of diagnostic spectral features warranted additional testing. A standard immersion image failed to show any zoning or features. By contrast, a DiamondView image clearly revealed parallel curved bands characteristic of synthetic origin. Quantitative UV-visible spectra exhibited strong broadband absorptions in the blue and green regions and created a transmission window in the yellow spectral region (figure 1). Detailed chemical analysis via LA-ICP-MS was performed to identify chromophores and color origin. The elements Mg, Cr, and Ni were detected at trace levels (a few ppma). Naturally occurring elements such as Ga were not detected, further indicating synthetic origin. Trace Cr was confirmed both by red fluorescence and by laser photoluminescence spectra with 514 nm excitation, exhibiting a doublet at 692/694 nm and side bands.

A combination of trace-element analysis and UV-visible spectroscopy clearly indicated that the yellow color originated from the much more effective chromophore known as “trapped holes,” associated with the trace amount of Mg and Cr in this stone (J.L. Emmett et al., “Beryllium diffusion of ruby and sapphire, Summer 2003 G&G, pp. 84–134). By comparison, a sapphire colored by Fe3+ would only display a pale yellow coloration with a concentration above 500 ppma. This synthetic sapphire, however, showed no iron above the detection limit ( 1 ppma). The contribution of trace amounts of Ni is not well known.
Natural and synthetic sapphires colored by a trapped-hole mechanism often possess an orange or reddish orangy hue. This synthetic sapphire exhibited a yellow coloration much like those of samples colored by Fe3+. This example demonstrates that a clear understanding of chromophore contribution and the application of relevant advanced testing can reliably identify the cause of color as well as natural or synthetic origin. In this instance, the prominent curved growth supported the identification, but even without this feature a thorough understanding of the cause of color in sapphire can provide helpful identification clues.