Kyanite is a gemstone quality aluminum silicate sometimes referred to as disthene, rhaeticite or cyanite. Its name is derived from the Greek word 'kuanos' or 'kyanos', which means 'deep blue', alluding to its typical blue color. However, in addition to blue, kyanite can also be colorless, white, gray, green, orange or yellow. With kyanite, color is typically not consistent throughout. In fact, most stones exhibit areas of light to dark color zones, along with white streaks or blotches. Kyanite is a polymorph of both andalusite and sillimanite. Polymorph gemstones share similar chemical compositions, but they exhibit different crystal structures. Kyanite forms as triclinic crystals, while andalusite and sillimanite are both orthorhombic.

Kyanite's most distinctive trait is its strong anisotropic properties. Most gems are anisotropic to some extent, but kyanite is the most famous of anisotropic gemstones. Anisotropic gemstones exhibit varying properties, depending on their crystallographic direction. When kyanite is cut parallel to its long axis, its hardness can range from 4 to 4.5 on the Mohs scale, but when cut perpendicularly, its hardness can range from 6 to 7.