Semi-Precious Gemstones: Kyanite
Kyanite used in Jewelry
Source: Austria, Brazil, India, Kenya, Myanmar, Serbia, Switzerland, USA, Zimbabwe
Kyanite derives its name from the ancient Greek word kyanos, meaning "blue," and the color of faceted transparent kyanite is very similar to the deep "cornflower blue" found in some varieties of sapphire.
Kyanite (disthne French, kyanit German, cianite Italian) is a silicate mineral in the aluminosilicate series of nesosilicates, commonly found in aluminium-rich metamorphic pegmatites (blueschist facies basalt) or within sedimentary rock. Kyanite forms elongated columnar crystals known as "blades," which are heavily striated lengthwise along the c-axis. Kyanite typically occurs in association with, or is a polymorph of several other silicate minerals such as andalusite, micas, sillimanite, quartz, staurolite, and talc.
One of the unique features of kyanite is that it is anisotropic (anisotropism), meaning that it exhibits variable Mohs hardness ratings along different crystallographic axes. The hardness rating of kyanite is a respectable 6 to 7 on the Mohs scale when scratch-tested along its a-axis or b-axis, but is a low 4 to 4.5 when scratch-tested lengthwise, along its c-axis. This characteristic must be considered when determining the final orientation of the cut stone within the rough crystal.
While transparent specimens can be faceted, opaque examples are cut as beads or cabochons. The color of kyanite is also not consistent throughout the crystal, which can show zoning and streaks of white, light "denim blue," and rusty reddish-orange color.
Kyanite Crystallography, Chemistry, Physical Properties
Kyanite Optical Properties
Along with being used as a gemstone, kyanite is used in serveral industrial and manufacturing applications from abrasives to ceramic products and dish-ware, and as a component in porcelain plumbing fixtures and electrical insulators. Kyanite is also used as a refractory material for high-temperature furnaces, kilns, incinerators and reactors. When heated to 1370C, kyanite converts to mullite (porcelainite). Kyanite is an "index mineral" which is used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.
Bibliography and Reference on Kyanite
1. Paul R. Shaffer, Rocks, Gems and Minerals  . Martin's Press
2. G. Simandl, K. Hancock, M. Rotella, Andalusite & Kyanite in Canada . Martin's Press
3. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition
4. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press
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