Micro-World Gems & Gemology, Winter 2017, Vol. 53, No. 4

Aragonite Spheres in Ethiopian Opal

Inclusion scene of pyrite crystals and aragonite spheres in Ethiopian opal.
Figure 1. This Ethiopian opal contained an interesting inclusion scene consisting of a string of octahedral pyrite crystals and aragonite spheres. Photomicrograph by Charuwan Khowpong; field of view 1.40 mm.

Recently, the authors examined an 88.51 ct pear-shaped white precious opal that displayed typical digit patterns and hydrophane behavior (see Fall 2013 Lab Notes, pp. 175–176). These properties were consistent with opal from the Wollo mining area in Ethiopia (see B. Rondeau et al., “Play-of-color opal from Wegel Tena, Wollo Province, Ethiopia,” Summer 2010 G&G, pp. 90–105). The stone also showed weak whitish blue fluorescence in long-wave UV and was inert to short-wave UV. No phosphorescence was observed.

Pyrite crystal among aragonite inclusions.
Figure 2. A single pyrite crystal is seen among spheres and clusters of translucent aragonite inclusions, both of which were identified by Raman spectroscopy. Oblique fiber-optic illumination. Photomicrograph by Charuwan Khowpong; field of view 1.05 mm.

In addition to its attractive digit pattern with spectral play-of-color, this opal hosted a number of unusual inclusions: opaque black octahedrons and translucent spheres. When viewed with a fiber-optic light, the opaque black octahedrons appeared a dark brassy metallic yellow (figure 1). Raman analysis identified them as pyrite, which has been previously reported in Ethiopian opal (N. Renfro and S.F. McClure, “Dyed purple hydrophane opal,” Winter 2011 G&G, pp. 260–270). Of particular interest is that the translucent spherical inclusions (figure 2), which formed as isolated spheres and clusters, were identified by Raman analysis as aragonite. The presence of aragonite spheres in opal is quite uncommon and has not been previously reported in precious opal from Ethiopia.

Ratima Suthiyuth is a senior staff gemologist, and Charuwan Khowpong is a colored stones analyst, at GIA in Bangkok.