Generally kyanite is closer to gray-blue, with a pearly or glassy luster. The color is often uneven, as in this specimen. It has two good cleavages. An unusual feature of kyanite is that it has Mohs hardness 5 along the length of the crystal and hardness 7 across the blades. Kyanite occurs in metamorphic rocks like schist and gneiss.
Kyanite is one of three versions, or polymorphs, of Al2SiO5. Andalusite and sillimanite are the others. Which one is present in a given rock depends on the pressure and temperature that the rock was subjected to during metamorphism. Kyanite signifies medium temperatures and high pressures, whereas andalusite is made under high temperatures and lower pressures and sillimanite at high temperatures. Kyanite is typical in schists of pelitic (clay-rich) origin.
Kyanite has industrial uses as a refractory, in high-temperature bricks and ceramics such as those used in spark plugs.
