Chemical composition -- Aluminum silicate plus iron -- Al2SiO3 + Fe.
Color -- reddish brown, pink, yellowish, greenish.
Optics -- R.I 1.63-1.65. Strongly pleochroic, often showing olive green to flesh-red. Because the reddish and greenish color combination is slightly reminiscent of the color change seen in alexandrite , andalusite has sometimes been referred to as "poor man's alexandrite," but there is no real color change under different light sources.
Durability -- Hardness 6.5 to 7.5.
Crystal structure -- orthorhombic.
Specific Gravity -- 3.13-3.17.
Sources -- Brazil is the primary source. Gems are usually under 5 carats; stones over 10 carats are quite rare. An opaque variety of andalusite known as chiastolite is often cut in cross section to reveal a black cross on a gray background.