Kyanite

Kyanite forms in metamorphic rocks under moderately high pressure and indicates deep burial of the host terrain. It occurs mainly as blades in schists and gneisses, often with garnet, quartz, mica and staurolite, and also in granite pegmatites. Notable sources of gem-quality material include Brazil (large blue and blue-green crystals), Kenya, Mozambique (dark blue), Nepal, Tanzania, Switzerland and several U.S. states such as North Carolina; it is also found in Austria, India, Italy, Madagascar, Myanmar, Namibia and Zimbabwe. South Africa, the United States, France and India are the leading producers of industrial kyanite.

Kyanite is recognized by its color, perfect cleavage, bladed crystals and strongly directional hardness. Blue stones show pronounced pleochroism (violet-blue, colorless and cobalt blue), and color zoning is common; a color change from greenish blue in daylight to purple in incandescent light and, rarely, chatoyancy have been reported. Kyanite is sometimes presented as a look-alike for blue sapphire, but its refractive index, optic character and specific gravity differ.

Heating pale blue kyanite to about 1,200 °C may remove its color. Kyanite has been synthesized only for research; beads sold online as "synthetic kyanite" are most likely look-alike materials.

Because of its perfect cleavage and variable hardness, kyanite set in rings should have protective settings; earrings, pendants and brooches are better suited to it. Most kyanites are heavily included, so do not use steam or ultrasonic cleaners. Clean only with a soft brush, mild detergent and warm water.

Abraham Gottlob Werner named kyanite in 1789 from the Greek word kyanos, "blue", after the mineral's usual color. The French spelling "cyanite" was commonly used by mineralogists through much of the 19th and early 20th centuries. The mineral was also once commonly called "disthene", meaning "two strengths", a reference to its different hardness in different directions.