Background: Two unmounted faceted stones were submitted to the GIA Gem Trade Laboratory for Identification Reports. These reportedly came from a parcel of stones that had been purchased in Mexico as opal by our client. However, subsequent testing by the client using standard gem-testing methods revealed properties that were outside the reported ranges for opal.
Visual Appearance: One of the specimens, weighing 2+ carats, was a transparent red emerald-cut that might easily be mistaken for high-quality "cherry" opal while the other, 3.5+ carat specimen was a slightly less transparent orange oval modified brilliant resembling much of "Mexican" opal seen in the gem trade.
Gemological Properties: Our testing revealed the following properties (those of the red stone given first)--R.I.: 1.522, 1 480; S.G.: 2.64, 2.39; polariscope reaction: anomalous birefringence in the form of snake-like bands; U.V. fluorescence: both were inert to LWUV radiation and fluoresced a faint, chalky yellow to SWUV. Magnification revealed minute gas bubbles in both specimens.
FTIR Spectroscopy : The two specimens exhibited spectral features in the infrared that were very similar to each other: a very small, broad absorption peak at 4490 cm~l, a broad plateau with an absorption edge at 3558 cm~l and an absorption peak at 2775 cm~l, and very strong absorption from about 2250 cm~1 to 600 cm~ 1. These features are quite distinct from those we have documented in both natural and synthetic opal.
EDXRF Spectroscopy: Of greatest significance here, chemical analysis by energy-dispersive X-ray fluorescence revealed selenium, an element that we have not detected in natural opals.
Discussion: While the R.I. and S.G. of the red specimen were well above the general values reported the gemological literature for opal, those of the orange stone were only slightly higher than those reported for the upper ranges of opal. However, the type of opal from Mexico that these two samples resembled typically has below-average values, e.g., an R.I. of 1.44 or lower and an S.G. of about 2.00 2.10. Although a broad peak was present in the "water region" of the samples' FTIR spectra, natural opals (and some synthetics) we have examined are typically opaque in this region. EDXRF analysis revealed the presence of selenium, which is interesting in that many red to orange glasses, commonly referred to as "selenium glass," owe their color to the presence of cadmium selenide or cadmium sulfo selenide.
Source: GIA Gem Trade Laboratory, August 8, 1995