mineral
Black Tourmaline (Schorl) in Matrix
NaFe2+3Al6(Si6O18)(BO3)3(OH)3(OH)
Hardness: 7-7.5 (Mohs scale); Color: Jet black to dark brown; Luster: Vitreous to sub-metallic; Crystal Structure: Trigonal (striated prisms); Cleavage: Indistinct; Specific Gravity: 3.0-3.2
- Hardness
- 7-7
- Color
- Jet black to dark brown
- Luster
- Vitreous to sub-metallic
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Physical properties
Hardness: 7-7.5 (Mohs scale); Color: Jet black to dark brown; Luster: Vitreous to sub-metallic; Crystal Structure: Trigonal (striated prisms); Cleavage: Indistinct; Specific Gravity: 3.0-3.2
Formation & geological history
Formed in granitic pegmatites and metamorphic rocks through hydrothermal and pneumatolytic processes where boron is present. Can be found in rocks dating from Proterozoic to Cenozoic eras.
Uses & applications
Used primarily in jewelry and for specimen collecting. Industrially, it is used for its piezoelectric and pyroelectric properties in pressure gauges and optical instruments.
Geological facts
Black tourmaline is known as Schorl; it makes up about 95% of all tourmaline found in nature. It was historically used to pull ash out of clay tobacco pipes because of its electrostatic properties when heated.
Field identification & locations
Identify by its characteristic vertical striations along the prism faces and its triangular/hexagonal cross-section. Commonly found in Brazil, Namibia, and the USA (California/Maine). It is a staple for collectors of rough mineral jewelry.
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