With a range of blue hues in a single stone, kyanite is a fabulous gem for beautiful and unique jewelry.  Kyanite has a particularly unique and interesting property: it has two  different measures of hardness in the same crystal.  A gem’s hardness is defined as its resistance to scratching and is usually stated as a single number or small range; diamond is 10 on the Moh’s hardness scale, quartz is 7.  When you look at a kyanite crystal,  it may appear to have a grain like a piece of wood; the hardness can be 5-5 1/2 in the direction of the grain and 7 across it.

Kyanite is named from the Greek word for dark blue, kyanos, from which we get “cyan” as a shade of blue.  Most kyanite is blue, but it can also be white, gray, orange, yellow and green.  Although still not very common in jewelry, good quality blue and green material is used for beads and cut stones.

Kyanite crystals grow in long flat “blades”.  The gem can be translucent to opaque and the color is often very unevenly zoned over the crystal.  It is a metamorphic mineral with chemical formula Al₂SiO_5.   It is also a “polymorph”; one of three minerals with the same chemical formula but different crystal structure (the other two minerals are andalusite and sillimanite).

Kyanite  can be found in many locations around the world including Austria, Burma, Brazil, Cambodia, India, Kenya, Myanmar, Nepal, Russia, Serbia, Switzerland, Tanzania, the United States and Zimbabwe.  Besides jewelry, kyanite is used in the manufacture of porcelain, spark plugs and  electronics.

Kyanite is fairly durable, but has perfect cleavage that can cause the stone to split with a single hard knock or blow. Don’t use ultrasonic cleaners, steamers, bleach or other harsh chemicals when cleaning kyanite; use water and a mild soap.

References:
Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York.
Gemological Institute of America, Gem Identification Lab Manual, 5/2012
http://www.gemselect.com/gem-info/kyanite/kyanite-info.php