mineral
Polished black tourmaline (probable schorl)
Schorl, NaFe²⁺₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
AI-generated identification · may be incorrectOpaque black, vitreous-to-resinous luster, commonly striated and prismatic; Mohs hardness 7–7.5, specific gravity about 3.1–3.3. The rounded, glossy surface appears tumbled or polished; crystal structure is trigonal with no prominent cleavage.
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Physical properties
Opaque black, vitreous-to-resinous luster, commonly striated and prismatic; Mohs hardness 7–7.5, specific gravity about 3.1–3.3. The rounded, glossy surface appears tumbled or polished; crystal structure is trigonal with no prominent cleavage.
Formation & geological history
Forms mainly in granite pegmatites and hydrothermal veins, crystallizing from boron-rich fluids during late stages of igneous activity. Exact age and locality cannot be determined from the photograph.
Uses & applications
Used as a collector mineral, ornamental stone, and occasionally in jewelry; black tourmaline is also sold for decorative and metaphysical purposes. It has little structural or industrial use compared with common rock-forming minerals.
Geological facts
Schorl is the most common tourmaline species and is strongly piezoelectric and often pyroelectric. Heating or pressure can produce opposite electrical charges on its ends.
Field identification & locations
Look for a black, glassy-to-submetallic body, longitudinal striations, and a relatively heavy feel; a hardness test should not scratch it easily with a steel knife. Similar polished obsidian is usually smoother and lacks tourmaline’s crystal striations; confirmation requires testing or an unpolished surface.
