A mineral which is anisotropic has a hardness that varies depending on its crystallographic direction. Many minerals will exhibit a variable hardness on different crystal faces, due to slight variations in the crystals. But kyanite exhibits a marked variable hardness in the very same crystal. Kyanite is quite soft when cut parallel to the long axis of the crystal, with a hardness of only 4 to 4.5 on the Mohs scale. But it has a hardness of about 6.5 when cut perpendicular to the long axis of the crystal.
Kyanite is usually blue but it can also be white, gray or green. In gemstones, a sapphire-like blue is the most valuable color. However, the color is typically not consistent throughout the crystal and can be blotchy or in streaks. You will typically find kyanite cut as cabochons, but we have recently found some high quality material cut in facets (a challenging job for the cutter 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 but a different crystal structure with another mineral. 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 and high voltage electrical insulators and in the ceramic industry.
Kyanite deposits are found in Brazil, North Carolina and Georgia in the USA, Switzerland, Russia, Serbia, India, Kenya, Zimbabwe, Cambodia, Burma and Nepal.
- First Published: August-15-2008
- Last Updated: February-16-2011
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