If it’s blue* and bladed, it’s kyanite.

Kyanite is a polymorph of andalusite and sillimanite. These three minerals share the same formula, but are stable at different P/T conditions.

*Sometimes also if it’s white and bladed, or grey and bladed.

Physical Properties
Chemical formula Al2SiO5
Class Nesosilicate
Isolated tetrahedra
Crystal system Triclinic
Habit Bladed
Parallel or radiating aggregates
Color Blue to white
Hardness 5 to 7
Specific gravity 3.6
Cleavage Two prominent:
Perfect (100), good (010)
Fracture Uneven
Luster Vitreous
Transparency Transparent to translucent
Streak White
Optical Properties
PPL Typically colorless
May be weakly blue and pleochroic
High relief
XPL High relief
1st order interference colors
δ 0.016
after Perkins, 344-345

Kyanite in Hand Sample

Kyanite hand sample

Blue kyanite speckled with garnet and chalcopyrite

Kyanite quartzite

Scanning electron micrograph of kyanite

Kyanite in Thin Section

Thin Section GigaPans

Kyanite quartzite, plane polars

Kyanite quartzite, crossed polars

Kyanite, PPL

Kyanite, XPL

Further Reading

Kyanite at

Kyanite at