Kyanite

Hardness: Varies significantly by direction (anisotropic hardness); 4.5-5 parallel to the long crystal axis, 6.5-7 perpendicular to it. Color: Blue, white, gray, less commonly green, black. Luster: Vitreous to pearly. Crystal Structure: Triclinic. Cleavage: Perfect in one direction (pinacoidal). Specific Gravity: 3.53-3.67.

Kyanite is typically formed during regional metamorphism of clay-rich sedimentary rocks under high pressure and relatively low temperatures. It is commonly found in schists, gneisses, and pegmatites. Geological Age: Varies depending on the metamorphic event, ranging from Precambrian to more recent.

Industry: Used in the manufacture of refractory products (e.g., high-temperature ceramics, furnace linings) due to its stability at high temperatures. Jewelry: Cabochons, beads (as seen in the image), and occasionally faceted for collectors. Its unique blue color and fibrous appearance make it appealing. Construction: Less common, but sometimes used in specialized ceramic applications.

Kyanite is one of the 'andalusite group' of minerals, which also includes andalusite and sillimanite. These three minerals are polymorphs, meaning they have the same chemical composition (Al2SiO5) but different crystal structures, forming under different conditions of temperature and pressure. Kyanite's variable hardness, referred to as 'anisotropic hardness,' is a distinctive and unusual property.

Field Identification: Distinguished by its characteristic blue color, bladed or columnar crystal habit, and anisotropic hardness (can be scratched by a knife parallel to the length but not perpendicular). Its perfect cleavage also aids identification. Common Locations: Significant deposits are found in Brazil, Nepal, Switzerland, Kenya, Myanmar, and parts of the USA (e.g., North Carolina, Georgia). Tips for Collectors: Look for well-formed bladed crystals in metamorphic rocks. Be aware of its variable hardness when working with specimens.