Gem-quality spinel (MgAl2O4) occurs in a variety of colors based on the trace elements present within the stone. While synthetic spinels are available in almost any color, some colors are rarely found in natural spinel. The New York lab received a 2.54 ct light yellowish green spinel with unusually strong green fluorescence (figure 1). This variety of color, along with the strong fluorescence (in both long-wave and short-wave UV radiation) is rare in natural spinel, and we needed proof that this stone was not synthetic.
A refractive index of 1.715 suggested the stone might be natural (flame-fusion synthetic spinels typically have an RI of 1.728). Microscopic examination revealed a very small fingerprint shallow to the table facet. While not conclusively diagnostic for natural origin, it supported the possibility. When observed under cross-polarized filters, the stone revealed very little strain, more consistent with a natural spinel. To confirm natural origin, PL spectra and trace element chemistry data were collected.
The PL spectra were collected at room temperature, using 514 nm laser excitation. The sharp and defined chromium emission features, with the strongest peak at approximately 685.5 nm (figure 2), verified that the stone was natural and unheated (S. Saeseaw et al., “Distinguishing heated spinels from unheated natural spinels and from synthetic spinels,” 2009). Heat treatment typically broadens and shifts the position of PL peaks (a similar effect is seen in synthetic spinels). Using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), high concentrations of natural trace elements were measured—particularly lithium, gallium, zinc, and beryllium. This reinforced our finding that the spinel was natural.