Polycristalline Kyanite

first published in Facette 22 (February 2016)

Figure 1: Water-worn pebble and cut stone, both polycristalline kyanite.

The Swiss Gemmological Institute SSEF received two blue translucent samples, a water-worn pebble, and a faceted stone of 1.72 ct cut from the same piece (Figure 1). The material was bought in Arusha (Tanzania) in December 2013 by Mr. Farooq Hashmi.

Standard gemmological methods quickly revealed the anisotropic polycrystalline character of the material (always bright in the polariscope). Consequently, the RI value could only be determined approximately (1.72). Since the identity of the material was not evident after this initial standard testing, it was analysed by Raman spectroscopy, which immediately revealed a distinct kyanite spectrum. The blue colour of kyanite is linked mainly to an intervalence charge transfer (Ti4+ – Fe2+, both elements found in trace concentrations in our samples) through the replacement of two Al3+ ions on adjacent crystal sites (Platonov et al., 1998; Henn & Schollenbruch, 2012; Krzemnicki, 2013), a colouring mechanism well known also in metamorphic blue sapphires.

Figure 2: Treacle-like appearance in polycristalline kyanite due to grain boundaries.

Microscopic observation revealed an interesting treacle-like appearance (similar to polycrystalline hessonite) due to kyanite grain boundaries (Figure 2). To the author’s knowledge, this is the first time that polycrystalline kyanite of gem quality has been reported. Mr. Hashmi has not encountered any other examples of this material on previous or subsequent buying trips.