Oval mixed-cut kyanite, 4.55 cts, 11.3 x 8.1 mm, Nepal. © The Gem Trader. Used with permission.
Kyanite’s physical properties make it very difficult to facet. However, it’s a popular collector’s gem, especially in sapphire-blue colors.
Oval mixed-cut kyanite, 4.55 cts, 11.3 x 8.1 mm, Nepal. © The Gem Trader. Used with permission.
Kyanite’s physical properties make it very difficult to facet. However, it’s a popular collector’s gem, especially in sapphire-blue colors.
| Data | Value |
|---|---|
| Name | Kyanite |
| Colors | Blue, blue-green, green; also white, gray, yellow, pink, orange, nearly black. Color zoned in individual crystals. |
| Hardness | 4-7.5; varies with direction in single crystals. |
| Fracture | Splintery |
| Cleavage | Perfect 1 direction |
| Formula | Al2SiO5 |
| Crystallography | Triclinic. Crystals bladed, flattened and elongated; fibrous, massive. |
| Crystallographic Forms | |
| Refractive Index | 1.710-1.734 |
| Birefringence | 0.017 |
| 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. Biaxial (-), 2V= 82-83°. Some stones may be Biaxial (+). |
| Optic Sign | Biaxial +, Biaxial - |
| Luster | Vitreous; pearly on cleavage. |
| Specific Gravity | 3.53-3.68; gems usually upper end of range. Cr-kyanite = 3.67-3.70. |
| Enhancements | Heating may turn pale blue kyanites colorless. |
| Transparency | Transparent to translucent |
| Phenomena | Chatoyancy (rare), color change. |
| Typical Treatments | Heat Treatment |
| Etymology | From the Greek kyanos, meaning “blue.” |
| Occurrence | In schists, gneiss, and granite pegmatites. |
| Inclusions | Cleavage fractures, growth tubes, single-phase (liquid) inclusions, and crystals of a wide variety of gem species, such as: andalusite, apatite, calcite, garnet, muscovite, quartz, rutile, staurolite, tourmaline, and zircon. |
Kyanites in a quartz matrix, 11 x 9.1 x 7.6 cm (crystals measure over 6 cm), Minas Gerais, Southeast Region, Brazil. © Rob Lavinsky, www.iRocks.com. Used with permission.
Kyanite is an aluminum silicate mineral that’s notoriously difficult to facet because of its perfect cleavage and variable hardness, even within the same crystal. When cut parallel to its (long) c-axis, it has a hardness of 4 to 4.5. When cut perpendicular to its c-axis, it has a hardness of 6 to 7.5.
Kyanite is polymorphous with andalusite and sillimanite. These gems share the same chemistry but have different crystal systems.
Faceted kyanites as display pieces or jewelry stones should appeal to collectors of unusual gems or aficionados of the art of gem cutting. If faceted by expert gem cutters, kyanites can make beautiful gemstones. However, they will typically contain many flaws and inclusions, especially in larger sizes.
Kyanites: Brazil (4.55, 7.80). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Blue kyanites are popular with collectors, but these gems can occur in many other colors, including green, pink, yellow, and orange.
Rectangular step-cut kyanite, 1.26 cts, 7.1 x 4.7 mm, Loliondo, Tanzania. © The Gem Trader. Used with permission.
Some stones may have color zones. Color change in some stones, from greenish blue in daylight to purple under incandescent light, has also been reported. In rare cases, cabbed kyanites may display chatoyancy.
Cat’s eye kyanite, 6.79 cts, 11.1 mm, Brazil. © The Gem Trader. Used with permission.
Due to their variable hardness, kyanites in rings should have protective settings. These gems make great options for other jewelry uses, such as in earrings, pendants, and brooches.
Some kyanites have been presented as blue sapphire lookalikes. Although some specimens do display comparable colors, gemologists can readily distinguish these gems. Sapphire and kyanite have different refractive indices, optic character, and specific gravity values. Of course, kyanite’s hardness values also distinguish it from sapphire. However, scratch testing isn’t recommended on finished gemstones.
If you’re considering a kyanite as a sapphire substitute, please keep in mind that kyanite is less durable than sapphire (but also much less expensive).
Kyanite. Photo © Joel E. Arem, PhD, FGA. Used with permission.
In the 1970s, scientists successfully synthesized kyanites for research into their coloration. However, the challenges kyanite poses to gem cutting make the chances for a viable jewelry market for a synthetic version very improbable. Furthermore, kyanite isn’t a rare mineral. It has many industrial uses, in fields such as abrasives, automotive brake materials, ceramics, and electronics.
Nevertheless, you might find so-called “synthetic kyanites” for sale online, especially as beads. Most likely, these are just lookalikes rather than lab-created kyanites. These may be instances of the term “synthetic” being used in the popular sense of “not real.”
Heating pale blue kyanites to 1,200º C may cause them to lose their color.
Many localities around the world produce kyanites. Some of the most notable sources of gem-quality material include the following:
Kyanites: Brazil (3.28); Kenya (2.60). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Chromium-vanadium bearing kyanite, rectangle cut, 1.96 cts, 9.1 x 5.8 mm, Tanzania. © The Gem Trader. Used with permission.
Faceters have cut gems up to about 20 carats. Stones over 5 carats are seldom completely clean. Many of these stones are Brazilian; some are African.
Since most kyanites are heavily included, don’t clean these gems with mechanical cleaning systems, like steam and ultrasound. Clean them only with a soft brush, mild detergent, and warm water. Consult our gemstone jewelry cleaning guide for more recommendations.
Sterling silver earrings with kyanite and white and blue topaz. Photo courtesy of liveauctioneers.com and Dalshire International.
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