Blue-gray kyanite crystal

Typically bladed and elongated, with blue-gray color, vitreous to pearly luster, perfect cleavage in one direction, and hardness Mohs 4.5 parallel to blades and 6.5–7 across them; specific gravity is about 3.5–3.7. The pictured fragment is too small for confident identification and could also be another blue-gray mineral.

Forms mainly in aluminum-rich metamorphic rocks, especially schist and gneiss, during medium- to high-pressure regional metamorphism. Most specimens formed hundreds of millions to over a billion years ago, depending on locality.

Used as a refractory and heat-resistant ceramic raw material; attractive transparent crystals may be cut as gemstones, though faceting is difficult because of variable hardness.

Kyanite is a polymorph of andalusite and sillimanite: all share the formula Al₂SiO₅ but form under different pressure-temperature conditions.

Look for blue bladed crystals with strong lengthwise striations and distinctly variable hardness; common localities include Nepal, Switzerland, Brazil, Kenya, and the United States. A hardness test and host-rock context are needed to confirm it.