Micro-World Gems & Gemology, Winter 2018, Vol. 54, No. 4

Lepidocrocite in Boulder Opal

Boulder opal with layer of lepidocrocite inclusions
Figure 1. This boulder opal contains a layer of needle-like lepidocrocite inclusions along the interface between the opal and the ironstone matrix. Photomicrograph by Nathan Renfro; field of view 0.96 mm.

The term “boulder opal” is used to describe gem-quality opal that fills in the pore spaces and cracks of its ironstone host. When cut, some of that ironstone matrix is included in the finished stone, often to add structural support to del­icately thin veins of opal (R.W. Wise, “Queensland boulder opal,” Spring 1993 G&G, pp. 4–15). The authors recently examined a boulder opal, presumed to be from Australia, that showcased some interesting inclusions along the interface between the opal and ironstone matrix. Microscopic observation revealed a carpet of minute needle-like inclusions radiating outward from the ironstone into the surface opal layer (see above). Where some of the inclusions broke the surface, they showed a submetallic luster in reflected light. In order to identify these unusual inclusions, we used Raman spectroscopy to analyze some of the surface-reaching needles. Raman spectroscopy and comparison with the reference spectra from the RRUFF mineral database showed that these inclusions were the iron hydroxide mineral lepidocrocite. Energy-dispersive X-ray fluorescence (EDXRF) testing revealed only the presence of iron, which was consistent with the Raman results. This is the first time the authors have encountered a lepidocrocite inclusion in boulder opal.

Nathan Renfro is manager of colored stones identification at GIA in Carlsbad, California. Bona Hiu Yan Chow is a staff gemologist at GIA in Hong Kong.