mineral
Tumbled black stone, likely black tourmaline
Schorl tourmaline, NaFe²⁺₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
AI-generated identification · may be incorrectDark black to brown-black, vitreous to resinous luster, commonly striated and elongated; Mohs hardness 7–7.5, no prominent cleavage, trigonal crystal structure, specific gravity about 3.1–3.3. The rounded surface indicates tumbling or polishing.
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Physical properties
Dark black to brown-black, vitreous to resinous luster, commonly striated and elongated; Mohs hardness 7–7.5, no prominent cleavage, trigonal crystal structure, specific gravity about 3.1–3.3. The rounded surface indicates tumbling or polishing.
Formation & geological history
Forms mainly in granite-related pegmatites and metamorphic hydrothermal veins as magma cools and boron-rich fluids crystallize; specimens range from Precambrian to younger Cenozoic deposits. The image alone cannot establish its exact species or age.
Uses & applications
Used as a collector mineral and gemstone, often marketed for ornamental carvings and cabochons; tourmaline is also valued for piezoelectric and pyroelectric properties. Common tumbled pieces generally have modest commercial value.
Geological facts
Black tourmaline is the most common tourmaline variety. Its strong longitudinal striations and ability to become electrically charged when heated or rubbed are diagnostic features.
Field identification & locations
Common sources include Brazil, Afghanistan, Pakistan, Madagascar, Namibia, and the United States, especially Maine and California. Confirm with hardness, striations, density, and a gemological test; iron-rich amphibole or slag can look similar.
