Fibrolite (Sillimanite)

Chemistry: Aluminum silicate
Crystal system: Orthorhombic; long slender prisms without distinct termination; often in parallel groups; also massive.
Color: Transparent to translucent; blue to blue/green, brown; phenomena: frequently fibrous giving cat’s eye.
Hardness: 6 - 7.5
Cleavage: Perfect: 1 direction, parallel to one long prism face.
Specific gravity: 3.25
Refractive index: 1.658 – 1.678; Biaxial positive; 0.02
Luster: Vitreous to silky.
Dispersion: Low.
Dichroism: Strong: pale green/dark green/blue.
Occurrence: Schists, gneisses and granites; Burma, Sri Lanka, USA.

Notes
Frequently fibrous; alternate name Sillimanite (after B Sillimand, a one-time professor at Yale University, USA); often reserved for fibrous massive variety found in Idaho, USA; polymorphous with andalusite and kyanite; fluorescence: weak red in transparent blue material; indistinct spectral bands in blue 462, 441, 410nm; faceted and cat’s eye (cabochon).