mineral
Black tourmaline (polished schorl)
Schorl, NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
AI-generated identification · may be incorrectTypically black, opaque, and vitreous to subvitreous, with Mohs hardness 7–7.5 and specific gravity about 3.0–3.3. Tourmaline has a trigonal crystal structure and generally indistinct cleavage; the pictured piece is polished or tumbled.
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Physical properties
Typically black, opaque, and vitreous to subvitreous, with Mohs hardness 7–7.5 and specific gravity about 3.0–3.3. Tourmaline has a trigonal crystal structure and generally indistinct cleavage; the pictured piece is polished or tumbled.
Formation & geological history
Forms mainly in boron-rich granite pegmatites and hydrothermal veins during late-stage igneous crystallization, commonly in Precambrian to younger rocks. It is often associated with quartz, feldspar, and mica.
Uses & applications
Used as a gemstone, ornamental stone, mineral specimen, and in some electrical or thermal applications because tourmaline is piezoelectric and pyroelectric.
Geological facts
Schorl is the most common tourmaline species and gains its black color mainly from iron. Rubbing or heating tourmaline can produce electrical charge and attract light particles.
Field identification & locations
Identify it by its black color, glassy polish, strong durability, and commonly lengthwise striations or triangular cross-section; hardness testing and crystal form help distinguish it from obsidian. Common sources include Brazil, Afghanistan, Pakistan, Madagascar, and the United States.
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Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
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