Chemistry: Hydrated aluminum silicate.
Crystal system: Orthorhombic; cross-shaped, interpenetrant twins at 60º or 90º;crystals display pseudo-hexagonal cross section.
Color: Transparent to opaque: reddish brown.
Hardness: 7 – 7.5
Cleavage: Poor: 1 direction; fracture: brittle, conchoidal.
Specific gravity: 3.65 – 3.78
Refractive index: 1.739 – 1.762; Biaxial positive; 0.011-0.015
Luster: Vitreous to resinous.
Dispersion: Moderate.
Dichroism: Colorless, yellow/red, golden yellow; varies.
Occurrence: Metamorphic; Switzerland, France, Brazil, Russia, Scotland, USA.
Notes
Most specimens are opaque and valued for its cross-shaped twins; also known as cross stones, fairy stones; faceted.
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