Kyanite

Hardness: 4.5-5 (parallel to length), 6-7 (perpendicular to length) on Mohs scale; Color: typically blue, but can be white, gray, green, black, or colorless; Luster: Vitreous to pearly; Crystal structure: Triclinic; Cleavage: Perfect in one direction, good in another; Specific Gravity: 3.53-3.67

Kyanite is a metamorphic mineral typically formed during regional metamorphism of clay-rich sedimentary rocks under high pressure but variable temperature. It is indicative of high-pressure metamorphic environments and can be found in schists, gneisses, and pegmatites. Its geological age varies depending on the metamorphic event it formed within.

Kyanite is used in the manufacturing of high-refractory products like porcelain plumbing fixtures, spark plugs, and brake linings due to its stability at high temperatures. Gem-quality kyanite is used in jewelry, often cut into cabochons or faceted stones, for its appealing blue color. While not widely used in construction, its refractory properties are valuable in certain specialized applications.

Kyanite is known for its anisotropic hardness, meaning its hardness varies significantly depending on the direction it is scratched. This property makes it quite unique among minerals. It is also one of the 'andalusite-kyanite-sillimanite' polymorphs, all sharing the same chemical formula (Al2SiO5) but having different crystal structures due to varying temperature and pressure conditions during formation.

In the field, Kyanite is often identified by its characteristic blue color, bladed or elongated crystal habit, and anisotropic hardness (a knife blade will scratch it easily along its length but not across it). It's commonly found in metamorphic rock formations worldwide, with significant deposits in Brazil, the United States (North Carolina, Georgia), Switzerland, Austria, India, Kenya, and Myanmar. When identifying it, look for its distinct bladed crystals in schistose or gneissic rocks. The presence of other metamorphic minerals like garnet or staurolite can also be a clue.