Study Helps Identify
An article by Ulrich Henn and Klaus Schollenbruch titled “Saphirblauer Disthen (Kyanite) aus Nepal [Sapphire-blue kyanite from Nepal]," published in Gemmologie: Zeitschrift der Deutschen Gemmologischen Gesellschaft (Vol. 61, No. 3, 2012, pp. 91–98), describes the properties of kyanites from two occurrences discovered in 1995 in Nepal and compares them with gems from other deposits. Nepalese kyanite has been produced since 1999 in small-scale mining operations near Daha and Suneri and near Jajarkot and Barah, all west-northwest of Katmandu. Along with material for cabochons and beads, greater quantities of calibrated and larger faceted stones have appeared in the market in recent years. Only a very small percentage of the material is facetable.
The bulk of the article presents the findings of the author's investigation of kyanites from Nepal. While the nx and nz values of the refractive index lie within the range of kyanites from other sources, the ny value is distinctly lower. These are in accordance with a biaxial positive optical character, whereas kyanite is normally described as biaxial negative. The specific gravity of 3.65 to 3.68 g/cm3 is also equivalent to that of stones from other occurrences.
The chemical composition, determined by EDAX, showed trace contents of iron, vanadium and titanium in concentrations similar to those in stones from other sources. The blue color is due to the iron and titanium contents, while the vanadium does not seem to influence the color. Traces of chromium appear to be responsible for the fluorescence of some stones. The authors also discuss the absorption spectra and the mechanisms that lead to different colors as well as color change and fluorescence.
Microscopic inspection reveals the whole spectrum of phenomena and inclusions that are also found in kyanites from other sources: large cleavage fractures, growth tubes, liquid inclusions and black crystals. With oblique illumination, all of these inclusions can produce iridescence. Additionally, many kyanites show strong color zoning.
While the introduction of the article is just a summary of general information familiar to every gemologist, the results of the investigation are a welcome addition. With the different optical characters and ny values, it is even possible to separate Nepalese kyanites from those of other sources.