Andalusite has a number of useful physical properties. It has the ability to withstand high temperatures without alteration. For that reason it is used to make high-temperature ceramics and refractories. The white porcelain of some spark plugs is made using andalusite.

Andalusite is one of a small number of minerals that commonly forms prismatic crystals with a square cross-section (see accompanying photo). This can be important information for identification in the field.

Transparent specimens of andalusite are often strongly pleochroic. This makes them have different apparent colors when viewed from different directions. This pleochroic effect allows andalusite to be cut into unique gemstones.

Although twinning is not common in andalusite, nicely crystallized specimens that possess twinning can be distinctive. Twinning can produce cross-shaped structures perpendicular to the crystallographic c-axis, similar to what is shown in the rock in the accompanying photo above.

Physical Properties of Andalusite
Category	Silicate mineral
Chemical Composition	Al2SiO5
Crystal System	Orthorhombic
Color	Greenish brown, yellowish green, pinkish brown, orangy brown, white, gray. Strongly pleochroic, often with yellowish green and orangy brown colors.
Streak	White
Luster	Vitreous
Diaphaneity	Transparent to nearly opaque
Cleavage	Poor to good in one direction
Fracture	Uneven to conchoidal with vitreous fracture luster
Tenacity	Brittle
Mohs Hardness	6.5 to 7.5
Specific Gravity	3.17
Crystal Habit	Usually massive. Crystals are prismatic with a nearly square cross section, sometimes in columnar aggregates.
Refractive Index	1.630 to 1.643
Diagnostic Properties	Strongly pleochroic. Prismatic crystals with a nearly square cross-section. Associated with kyanite, sillimanite, corundum, or cordierite.
Uses	Used to make heat-resistant bricks and ceramic objects used in the steel-making and metal processing industries. Used to make chemicals, glass and ceramic products. Specimens with exceptional clarity and color are cut into gems.