Kyanite is an aluminium silicate of formula Al₂SiO₅, one of three polymorphs of aluminium silicate alongside andalusite and sillimanite. The three minerals share an identical chemical composition and differ only in crystal structure and the conditions of pressure and temperature under which they form. Kyanite is the high-pressure, low-to-moderate-temperature polymorph, characteristic of metamorphic rocks formed at depth in collisional orogenic belts.

Properties

Kyanite crystallises in the triclinic system, in long bladed crystals with two near-perpendicular cleavage directions. Its most distinctive physical property is strongly anisotropic hardness: along the c-axis (the long direction of the blade) the Mohs hardness is approximately 4 to 4.5, while perpendicular to the c-axis it rises to approximately 6 to 7. This disthene property (from the Greek for two strengths) is the source of the mineral's older synonym disthene. A jeweller can draw a steel needle along the long axis of a kyanite crystal and scratch it, then turn the crystal ninety degrees and find the same steel cannot mark it.

Refractive indices range from 1.71 to 1.74, with a birefringence of about 0.016. Specific gravity is 3.65 to 3.69. The mineral is biaxial negative with strong pleochroism in coloured material: blue or violet along one direction and near-colourless along another in typical Kenyan or Brazilian blue stones.

Colour and varieties

Kyanite is best known in saturated blue, the colour produced by trace iron and titanium with charge transfer mechanisms similar to those colouring blue sapphire. Colour ranges from pale icy blue through mid-blue to deep saturated electric blue, with a small fraction of stones reaching what the trade calls cornflower or peacock intensity. Other colours include green (vanadium- or chromium-coloured, sometimes called green kyanite or trade-named tiffany kyanite), grey, colourless, and rare orange (manganese-coloured, found in Tanzania and the United States).

Cat's-eye kyanite occurs occasionally in material with parallel inclusions oriented along the c-axis. Star kyanite is rare. The most highly valued material is medium to deep blue, clean, and large enough to face up well despite the modest hardness.

Sources

Major modern sources include Nepal (the Mount Everest region, which has produced exceptional saturated blue gem material since the 1990s), Kenya (the Taita-Taveta region near Voi, source of a large modern supply of blue and bicolour kyanite), Brazil (Minas Gerais), Tanzania (Umba Valley and the Loliondo area for orange material), Switzerland (the historical type locality, in modest quantities for collector material), and the United States (with North Carolina, Vermont and New Hampshire producing collector and ornamental specimens).

Cutting and setting

The directional hardness and the perfect cleavage make kyanite a notoriously difficult stone to cut and to set. Lapidaries must orient cutting and polishing operations relative to the c-axis to avoid working with the soft direction, and bezels must be designed to avoid pressure across the cleavage. The mineral is brittle, sensitive to thermal shock, and unsuited to ultrasonic cleaning, steam cleaning, or torch repair without removal of the stone.

For these reasons kyanite has remained a collector and connoisseur stone rather than a mainstream commercial gem despite its frequently spectacular colour. Cabochon cutting is the most common form for ornamental and lower-value material; faceted stones are usually emerald cut, oval, or cushion shapes that allow the cutter to favour the harder direction. Setting is best undertaken in protected mountings (low bezels, recessed pavé settings) rather than in elevated solitaires.

Identification

Kyanite is straightforward to identify on standard gemmological instruments: refractive indices in the 1.71 to 1.74 range, biaxial negative optic character, density above 3.6, distinctive directional hardness verifiable on rough or unfinished surfaces, and (for blue material) a characteristic absorption pattern with iron-related bands. Pleochroism is strong and visible to the unaided eye in larger stones.

Trade and market

Kyanite is moderately priced relative to other blue stones of comparable saturation: top-quality faceted Nepalese kyanite of clean clarity and strong blue colour reaches the low hundreds of dollars per carat at retail in the small-stone size range, with very large or exceptional pieces commanding more. The principal market constraints are the cutting and setting difficulty, which limit broader retail adoption, and the relatively limited supply from Nepal and Kenya at top colour grades.

Industrial kyanite (used as a refractory mineral and as a source of mullite for ceramic and metallurgical applications) is a separate market entirely, with bulk production from various mid-Atlantic and southern US deposits and from Kenya and Brazil. Industrial-grade kyanite is generally not gem quality.