Kyanite

Kyanite rough crystal

Kyanite (also called cyanite and disthene) was named in 1789 from the Greek word kyanos, which means blue, its most common color. The blue color is caused by iron and titanium impurities. Kyanite can also be colorless, white, gray, green, yellow, and the recently found orange from Tanzania. As with many gems, kyanite's pure form is colorless. Until the turn of the 20th century, transparent blue crystals were often sold to Europeans as blue sapphire. Kyanite is transparent to translucent and typically grows in long and slender bladed crystals.   

Kyanite is one of three aluminosilicate minerals. These three minerals--kyanite, andalusite, and sillimanite--are polymorphs, meaning they have the same chemistry but form different crystal structures depending on heat and pressure. Kyanite forms a triclinic crystal, while andalusite and sillimanite are both orthorhombic.

You're probably familiar with how much easier it is to split wood along the grain than across it. So it is with kyanite. The Mohs hardness along kyanite's long axis is 4 to 4.5, making it rather soft, while its hardness perpendicular to its axis ranges from 6 to 7. This "directional hardness" and kyanite's brittleness pose a challenge to the lapidarist when faceting gems. Given kyanite's variable hardness, care should be taken in storing and handling it. Treat it as you would pearls.

Kyanite is found in Myanmar, Brazil, Africa, Austria, Switzerland, and the United States. The best gem-grade kyanite comes from Nepal. Kyanite is not synthesized.

Image courtesy of minerals.net.

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