Kyanite is an aluminum silicate mineral best known for its blue color and for its unusual hardness. A kyanite crystal is about 4.5 to 5 on the Mohs scale when scratched along its length and about 6.5 to 7 when scratched across it. That split personality, together with perfect cleavage, makes kyanite one of the hardest gems to cut well. Fine transparent blue kyanite from Nepal became well known in the gem trade in the 2000s.
What kyanite is
Kyanite has the formula Al2SiO5. Two other minerals, andalusite and sillimanite, share exactly the same chemistry but have different crystal structures. Geologists call minerals like these polymorphs. Which of the three forms depends on the pressure and temperature where the rock formed. Kyanite is the high-pressure form, so its presence in a rock tells geologists about the depth of burial.
Kyanite forms in metamorphic rocks such as schist and gneiss, made from aluminum-rich sediments like shale that were buried and heated. It also occurs in some pegmatites and quartz veins. Crystals are long, flat blades, often bent or striated, with a pearly sheen on the cleavage faces.
The name comes from the Greek kyanos, "dark blue." Older books sometimes call it disthene, from Greek words meaning "two strengths," a nod to its directional hardness.
Directional hardness
Most minerals have about the same hardness in every direction. Kyanite does not. Its atoms are arranged in long chains, and the bonds along the length of the crystal are weaker than those across it.
- Along the length of a crystal (parallel to the long axis), kyanite is about 4.5 to 5 on the Mohs scale. A steel knife can scratch it.
- Across the crystal (perpendicular to the long axis), it is about 6.5 to 7. A knife cannot scratch it in that direction.
This is one of the best-known examples of anisotropic hardness. It is also a useful field test: a knife that scratches a blue blade along its length but not across it points strongly to kyanite. The page on the Mohs hardness scale explains how scratch hardness is measured, and the page on Vickers hardness testing covers indentation methods that can measure the difference.
Colors of kyanite
Blue kyanite
Blue is the classic color. It comes from small amounts of iron and titanium, similar to the cause of color in blue sapphire. The blue is often uneven, with deeper streaks or patches along the length of the blade and paler or nearly colorless areas in between. Strong color zoning is common.
Nepal kyanite
Transparent kyanite from Nepal became a regular gem trade item in the 2000s. The best Nepalese material has a saturated, even blue that some buyers compare to fine sapphire. It is often cleaner than kyanite from other sources and has made faceted kyanite far more common than before.
Other colors
Kyanite also comes in green, teal, orange, gray and colorless. Orange kyanite, colored by manganese, was found in Tanzania in the late 2000s. Teal and green kyanite come from Brazil, Kenya and elsewhere. Brazil and India have long produced blue blades, mostly for cabochons, beads and specimens.
Pleochroism and optics
Kyanite is pleochroic. It shows different shades of blue, from violet-blue to colorless or pale blue, when viewed from different directions. The page on pleochroism in gemstones explains this effect. Cutters orient the stone to show the deepest blue face up.
Kyanite's refractive index of about 1.71 to 1.73 is fairly high, which gives a well-cut stone good brilliance. It crystallizes in the triclinic system, the least symmetrical of the seven crystal systems, described on crystal systems in gemstones.
Cutting kyanite
Kyanite is among the most difficult gems for lapidaries. Several problems combine:
- Perfect cleavage along the length of the crystal means the stone can split if stressed, heated or struck.
- Directional hardness means facets cut at different angles to the crystal axis polish at different rates. One facet may polish easily while the next stays scratched or develops a rough finish.
- The blades are thin and flat, which limits the shapes and sizes that can be cut.
- Color zoning and pleochroism force the cutter to choose an orientation that shows good color while still avoiding cleavage planes.
Cutters work slowly, use fine abrasives, keep the stone cool and plan facet angles to stay clear of the cleavage directions. Even so, stones may break during cutting. Clean, well-cut faceted kyanite of good color is less common than its low rough price might suggest. The page on cleavage in gemstones explains how cutters handle stones like this.
Identifying kyanite
Kyanite's blue can resemble sapphire, tanzanite, iolite and blue tourmaline. Its directional hardness, pearly cleavage faces and bladed habit identify rough at a glance. For cut stones, a refractive index around 1.71 to 1.73 with moderate birefringence, along with its strong pleochroism, helps separate it. Sapphire has a higher refractive index and specific gravity. The gemological instruments page explains the tools used.
Most kyanite on the market is untreated.
Wearing and caring for kyanite
Because one direction is only 4.5 to 5, kyanite can scratch from ordinary dust in daily wear. Its perfect cleavage also makes it vulnerable to splitting from a knock. Pendants, earrings and brooches are the best uses. A kyanite ring should have a protective setting such as a bezel and should be worn with care.
Clean kyanite with lukewarm soapy water and a soft cloth. Avoid ultrasonic and steam cleaners and sudden temperature changes. Store it apart from harder gems. See gemstone durability for a wider comparison.
Buying kyanite
Look for even, saturated blue with minimal zoning and good transparency. Fine Nepal kyanite with sapphire-like color commands the highest prices. Check facet edges and surfaces for chips, scratches and a poor polish, since kyanite is hard to finish well. Opaque blue blades and cabochons are inexpensive and popular for beads and specimens.
Frequently asked questions
Why does kyanite have two hardnesses?
Its crystal structure has weaker bonds along the length of the crystal than across it. It is about 4.5 to 5 along the length and 6.5 to 7 across, depending on the direction of the scratch.
Is kyanite good for rings?
Kyanite scratches and splits more easily than most ring stones. It is better in pendants and earrings, or in a protective ring setting worn occasionally.
Where does the best kyanite come from?
Nepal is known for transparent, saturated blue kyanite. Brazil, India, Kenya and Tanzania also produce gem and specimen material.
Is kyanite treated?
Kyanite is almost always sold untreated.