Andalusite is one of many crystalline forms of aluminum silicate, occurring in contact metamorphic rock (Figure 1). It is mined commercially in the United States, Kazakhstan, and South Africa to make temperature-resistant and insulating porcelains.

 

Andalusite is usually pink but white, gray, yellow, green, and violet varieties also occur frequently. It crystallizes in the orthorhombic system. Hardness: 7.5.

 

Chiastolite

Chiastolite is a variety of andalusite, aluminium silicate (Al₂OSiO₄), found in metamorphic rocks (Figure 2). It has elongated prismatic crystals, which in cross-section show a black cross on a gray ground. Hardness 7.5; relative density 3.1–3.2.