Kyanite

Posted on August 31, 2022 by John J. Bradshaw, GIA GG

INTRODUCTION Kyanite is a mineral species crystallizing in the triclinic system with the formula AL2SiO5. There are two other minerals, sillimanite and andalusite, that have the identical chemical composition but distinctly different optical and physical properties. The name is from the Greek kayanos meaning blue. Kyanite can be extremely attractive as mineral specimens (Figure 1) and is actively sought after by collectors. It is well- known for being extremely difficult to facet due to one direction of perfect cleavage and variable hardness. Although many minerals have slight differences in hardness depending on crystallographic orientation, the difference in hardness of kyanite is extreme. Across the crystal, the hardness is 6.5- 7.5 while the hardness along the length is 4.5-5. LOCALITIES/COLORS/CLARITY Through the years, kyanite has been sourced from all seven continents. For the purposes of this gem note, three sources will be highlighted: Brazil, Nepal, and Tanzania. From any of these localities, only a small percentage of kyanite found is facetable. Brazil has produced a great deal of kyanite, most as mineral specimens but certainly facetable material as well. Colors range from light to medium blue (Figure 2), and sometimes green (Figure 3). In most stones, there are eye visible inclusions. Eye clean stones are exceedingly rare. Rich, intense blue kyanite from Nepal began hitting the mar- ket in the late 1990s (Figure 4) from several small deposits west/northwest of Katmandu. For several years, the supply was very good although that has significantly decreased in the last few years. Nepalese kyanite often displays color zones of medium and medium-dark blue (Figure 5). Overall, kyanite from Nepal is cleaner than its Brazilian counterpart. The last locality is Nani Hill, Loliondo, Arusha region in Tanzania. This deposit is significantly smaller than finds in Brazil or Nepal but remarkable in that the kyanites are a bright orange color (Figure 6). This deposit is relatively new, discovered only 12-15 years ago. Production is small and all faceted stones show eye visible inclusions. It remains to be seen if there will be any additional production with better clarity. FIGURE 2. An unusual green kyanite with a blue color zone from Brazil, 2.29 ct. Faceted and photo by John Bradshaw. www.rarestone.com STONE SIZES Brazilian material has yielded up to 20 ct but seldom clean over 5 ct. Nepalese kyanite also is typically seen in sizes less...

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