KYANITE: Brazil (4.55, 7.80). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Kyanite is very rare as a faceted gem, especially if free from inclusions and flaws. The material is extremely difficult to cut because of its perfect cleavage and the extreme variability in hardness in different directions in the same crystal. A few catseye kyanites are known to exist.
| Data | Value |
|---|---|
| Name | Kyanite |
| Colors | Blue, blue-green, green; also white, gray, yellow, pink, nearly black. Color zoned in individual crystals. |
| Hardness | 4-7.5; varies with direction in single crystals. |
| Fracture | Splintery |
| Cleavage | Perfect 1 direction |
| Stone Sizes | Gems have been cut up to about 20 carats: they are seldom completely clean over 5 carats, however Many of these stones are Brazilian; some are African. Smithsonian Institution (Washington, D.C): 10.7 (blue, Brazil): 9.1 Igreen. Brazil); 4 9 iblue, Tanzania). Private Collection: 6.57 (blue-green, North Carolina). Devonian Group (Calgary, Alberta, Canada): 14.0 (blue, Africai; 8.55 (bluish. Africa\. Royal Ontario Museum (Toronto, Ontario, Canada): 40.26, 12.38 (rectangular step-cut, Brazil. |
| Formula | Ak2SiO5. |
| Crystallography | Triclinic. Crystals bladed, flattened and elongated; fibrous, massive. |
| Refractive Index | 1.710-1.753 |
| Birefringence | 0.017 (Cr-kyanite up to 0.033). |
| Dispersion | 0.020. |
| Luminescence | Variable fluorescence, mostly dim red in LW. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long |
| Absorption Spectrum | One line observed in deep red at 7100 and 2 lines in deep blue, with dark edge at about 6000. |
| Pleochroism | Pronounced: violet-blue/colorless/cobalt blue. Also pleochroic in all shades of yellow-green and green. |
| Optics | a = 1.710-1.718; β = 1.721-1.723; γ = 1.727-1.734. Cr-kyanite: a = 1.714; β = 1.724; γ = 1.731; birefringence = 0.017; S.G.=3.67 birefringence = 0.017. a = 1.720; β = 1.730; γ = 1.753; birefringence = 0.033. Biaxial (-), 2V= 82-83°. |
| Optic Sign | Biaxial - |
| Luster | Vitreous: pearly on cleavage. |
| Specific Gravity | 3.53-3.68; gems usually upper end of range. Cr-kyanite = 3.67-3.70. |
| Transparency | Transparent to translucent. |
| Phenomena | Chatoyancy (rare), color change. |
Trimorphous with Andalusite, Sillimanite.
Optics: a = 1.710-1.718; β = 1.721-1.723; γ= 1.727-1.734.
Cr-kyanite: a = 1.714; β= 1.724; γ= 1.731; birefringence = 0.017; S.G.=3.67 birefringence = 0.017: S.G. = 3.67.
a = 1.720;,β= 1.730; γ= 1.753; birefringence = 0.033; S.G. = 3.70.
Biaxial ( – ), 2V= 82-83°.
Occurrence: in schists, gneiss, and granite pegmatites. Many localities are known.
Various places in the United States, especially Yancy County, North Carolina: deep blue or green crystals, up to 2 inches long, some facetable.
Vermont; Connecticut; Virginia; Georgia; Massachusetts. Mozambique: dark blue, with Cr and Ti.
India; Italy.
Brazil: large blue and blue-green crystals.
Machakos District, Kenya: large blue crystals, banded with green; also colorless!
Switzerland: with staurolite in schist.
Kenya: fine blue color, facetable.
Comments: Kyanite is very rare as a faceted gem, especially if free from inclusions and flaws. The material is extremely difficult to cut because of its perfect cleavage and the extreme variability in hardness in different directions in the same crystal. A few catseye kyanites are known to exist.
Name: From the Greek kyanos, meaning blue.
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