metamorphic
Staurolite (Cross Stone)
Staurolite (Fe2+2Al9Si4O23(OH))
Hardness: 7 to 7.5 on Mohs scale; Color: Dark brown, brownish black, or reddish brown; Luster: Vitreous to dull; Crystal structure: Monoclinic (pseudo-orthorhombic) often forming 60-degree or 90-degree cruciform twins; Cleavage: Distinct; Specific gravity: 3.7 to 3.8.
- Hardness
- 7 to 7
- Color
- Dark brown, brownish black, or reddish brown
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 to 7.5 on Mohs scale; Color: Dark brown, brownish black, or reddish brown; Luster: Vitreous to dull; Crystal structure: Monoclinic (pseudo-orthorhombic) often forming 60-degree or 90-degree cruciform twins; Cleavage: Distinct; Specific gravity: 3.7 to 3.8.
Formation & geological history
Formed through regional metamorphism of aluminous rocks like shale. It typically occurs in medium-grade metamorphic rocks such as mica schists and gneisses, often associated with almandine garnet, kyanite, and sillimanite.
Uses & applications
Primarily used as a mineral specimen for collectors and as a semi-precious stone for jewelry, especially in its natural 'cross' form. Historically used as a sandblasting abrasive, though mostly replaced by synthetic alternatives.
Geological facts
Commonly known as 'Fairy Stone' or 'Fairy Cross' due to its natural cross-shaped twinning. Legend says the stones are the tears of fairies who wept when they heard of the crucifixion of Christ. It is the official state mineral of Georgia, USA.
Field identification & locations
Identify in the field by its characteristic cruciform twinning and inclusion within mica schist matrices. Found in the Blue Ridge Mountains (USA), Brittany (France), and the Ural Mountains (Russia). Collectors look for well-defined 90-degree 'Greek' crosses.
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