metamorphic
Schist with Schorl (Black Tourmaline)
NaFe3Al6(BO3)3Si6O18(OH)4 (Schorl)
The crystal insert is black, vitreous to sub-metallic, and exhibits a trigonal crystal system with vertical striations. Hardness of the tourmaline is 7 to 7.5 on the Mohs scale. The host rock (matrix) is earthy and fine-grained, appearing to be a micaceous schist or mudstone.
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Physical properties
The crystal insert is black, vitreous to sub-metallic, and exhibits a trigonal crystal system with vertical striations. Hardness of the tourmaline is 7 to 7.5 on the Mohs scale. The host rock (matrix) is earthy and fine-grained, appearing to be a micaceous schist or mudstone.
Formation & geological history
Formed through regional metamorphism or in granitic pegmatites where boron and silicate-rich fluids crystallize. The host rock was originally sedimentary clay or silt transformed by heat and pressure.
Uses & applications
Black tourmaline is widely used in spiritual and metaphysical circles as a 'protection stone.' Industrially, its piezoelectric properties were used in pressure gauges, though it is now primarily a collector and specimen piece.
Geological facts
Tourmaline is unique because it is pyroelectric, meaning it can generate an electrical charge when it is heated or cooled. Famous large deposits are found in Brazil and Madagascar.
Field identification & locations
Identify in the field by looking for elongated, columnar crystals with a rounded triangular cross-section and distinct striations along the length of the crystal faces.
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