Chiastolite (Silicate-Neosilicate) 

Al2SiO5

Hardness: 7.5
Specific Gravity: 3.18

Characteristics: A variety of andalusite having a square cross section that displays a 'maltese cross' pattern resulting from carbonaceous impurities; Polymorph of kyanite and sillimanite; Red, brown, or green; vitreous luster; translucent to opaque; insoluble in most acids.

Found in low pressure aluminum-rich metamorphic rocks, commonly schists.

(Primary text: Perkins, D. 2002. Mineralogy. Prentice Hall.)