Staurolit

Staurolite is a striking nesosilicate mineral with the chemical formula (Fe,Mg)2Al9(Si O4)4(O H)2. It is best known for its distinctive cruciform or cross-shaped twinned crystals, which are highly sought after by collectors. These twins are typically polysynthetic, meaning they are composed of multiple intergrown crystals. The name staurolite is derived from the Greek word "stauros," meaning cross, referring to this characteristic twinning.

Staurolite crystals are usually prismatic and often occur as stout, opaque, reddish-brown to black prisms, sometimes exhibiting a faint striation parallel to the crystal length. They are generally found in metamorphic rocks such as schists and gneisses, forming under conditions of moderate to high pressure and temperature. Common associated minerals include garnet, kyanite, and micas.

While the classic cross-shaped twins are most famous, staurolite can also form as simple prisms without twinning. The color is generally consistent throughout the crystal, and it possesses a vitreous to dull luster. Staurolite is relatively hard, rating about 7 to 7.5 on the Mohs scale of hardness. It is not a particularly common mineral but is found in various locations worldwide, including parts of Europe, North America, and South America. Due to its unique appearance and historical significance, staurolite is a popular choice for mineral specimens and has also been used in some cultures as a talisman.