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By Reviewed By Andreas Zabczyk Aug 15, 2008 Updated Jan 23, 2019

Kyanite Hardness

Virtually every gemstone variety has some unique and interesting property. In the case of kyanite, a member of the aluminosilicate series, that property is something known as anisotropism.

Kyanite Crystals from Burma
Kyanite Crystals from Burma

A mineral that is anisotropic has different properties depending on its orientation. Many minerals exhibit variable hardness depending on their axes, due to slight variations in the crystals. Kyanite exhibits a marked variation in hardness. When it is cut parallel to its long axis, the hardness of kyanite can range from 4 to 4.5 on the Mohs scale, but when cut perpendicularly, its hardness can range from 6 to 7 in the same crystal.

Kyanite is usually blue but it can also be white, gray or green. In gemstones, a sapphire-like blue is the most valuable blue hue. However, the color is not usually consistent throughout the crystal and can be blotchy or in streaks. Kyanite is typically cut as cabochons, but some high quality material is faceted (a challenging job for the lapidary due to kyanite's variable hardness). Gem-quality kyanite has a vitreous to almost pearly luster. As a gemstone kyanite is always untreated.

Kyanite is a polymorph with two other minerals; andalusite and sillimanite. A polymorph is a mineral that shares the same chemistry with another mineral, but has a different crystal structure. For example, both kyanite and andalusite are aluminum silicates. But where kyanite has a triclinic crystal habit, andalusite is orthorhombic.

In addition to its use as a gemstone material, kyanite has a number of useful industrial applications, due to its stability at high temperatures. Kyanite is widely used in the manufacture of glass, burner tips, spark plugs, heating elements, high voltage electrical insulators and in the ceramic industry.

Kyanite deposits are found in Brazil, the USA (North Carolina and Georgia), Switzerland, Russia, Serbia, India, Kenya, Zimbabwe, Cambodia, Burma and Nepal.

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