Micro-World Gems & Gemology, Spring 2022, Vol. 58, No. 1

Hyalite with Magnificent Internal Features

Purple fluorite cube inclusion in a hyalite.
Figure 1. Purple fluorite cube, identified by micro-Raman spectroscopy, in a hyalite. Photomicrograph by U. Hennebois; field of view 1.2 mm.

The Laboratoire Français de Gemmologie (LFG) received for analysis a 1.83 ct hyalite, also known as opal-AN: an amorphous (A) opal containing hydrated silica molecules that are network forming (N) (E. Fritsch et al., “Green-luminescing hyalite opal from Mexico,” Journal of Gemmology, Vol. 34, No. 5, 2015, pp. 490–508). This is one of the rarest varieties of gem-quality opal. The gem fluoresced green under long-wave ultraviolet (365 nm) and, with more intensity, under short-wave UV (254 nm), due to U6+ in the form of uranyl (UO2)2+ (E. Gaillou et al., “Luminescence of gem opals: A review of intrinsic and extrinsic emission,” Australian Gemmologist, Vol. 24, No. 8, 2011, pp. 200–201). A purple fluorite (identified by a micro-Raman spectrometer; figure 1) and a series of cube-shaped cavities were visible (figure 2) in the hyalite, which appear to be all interconnected. Sometimes these cavities also contain small crystals of fluorite relics detected by Raman. Thus, these cavities may be the result of the partial dissolution of fluorite crystals. This would explain their cubic morphology, which cannot be explained by the dissolution of amorphous hyalite.

Partially dissolved fluorite relics in interconnected cavities.
Figure 2. Interconnected cavities observed in a hyalite containing partially dissolved fluorite relics identified by Raman spectroscopy. Curved growth layers are also observed in the background. Photomicrograph by A. Delaunay; field of view 2 mm.

Between crossed polarizers, high-order interference colors were observed (figure 3), as expected for this gem. This is due to anomalous birefringence linked with the strain present between the curved growth layers of hyalite. Also, a fluid inclusion was observed in the intersection of the botryoids (again, see figure 3). This inclusion is composed mostly of gas, suggesting that this hyalite was deposited from a gaseous phase. Although some of these features have been previously observed in hyalite, it is rare to see them all in the same sample. To the best of our knowledge, this is the first reporting of pronounced cubic cavities in hyalite.

High-order interference colors observed around a fluid inclusion.
Figure 3. This fluid inclusion contains mostly a gaseous phase. It is trapped at the intersection of curved botryoidal growth zones and presents high-order interference colors obvious between crossed polarizers. Photomicrograph by U. Hennebois; field of view 1 mm.

Ugo Hennebois, Aurélian Delaunay, and Stefanos Karampelas (s.karampelas@lfg.paris) are affiliated with the Laboratoire Français de Gemmologie (LFG) in Paris. Emmanuel Fritsch is with the University of Nantes in France.