Kyanite

Kyanite is remarkable for a hardness that changes with crystal direction, measuring about 4.5 parallel to the length of a crystal and roughly 7 across it. An aluminum silicate (Al2SiO5), it crystallizes in the triclinic system as bladed or columnar crystals with a vitreous luster, a colorless streak, perfect {100} cleavage, and a splintery fracture. Colors are typically blue, white, gray, or green, with a specific gravity between 3.53 and 3.65. Kyanite is an important metamorphic index mineral that signals high-pressure conditions, and its blue crystals are sometimes cut as gemstones. Its bladed crystals and perfect cleavage in one direction, combined with the directional hardness, make it distinctive.

El disthena es notable por tener una dureza variable que depende de la dirección de la muestra. Es un aluminosilicato de potasio que forma cristales azul pálido a azul intenso en rocas metamórficas de alto grado.