mineral
Black tumbled stone, possibly black tourmaline
Uncertain from the image; possibly schorl tourmaline, NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
AI-generated identification · may be incorrectSmall, dark, smoothly rounded specimen with a subdued luster; the photo does not show diagnostic crystal structure or cleavage. If schorl, hardness is 7–7.5 Mohs and specific gravity about 3.0–3.3.
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Physical properties
Small, dark, smoothly rounded specimen with a subdued luster; the photo does not show diagnostic crystal structure or cleavage. If schorl, hardness is 7–7.5 Mohs and specific gravity about 3.0–3.3.
Formation & geological history
Identification and age cannot be determined from the image. Schorl forms in boron-rich granitic pegmatites and metamorphic rocks, commonly during late-stage crystallization.
Uses & applications
Black tourmaline is used as a jewelry stone and mineral specimen; polished pieces are also sold for decorative collecting.
Geological facts
A polished pebble’s shape and dark color alone are not enough to distinguish tourmaline from obsidian, onyx, or other black minerals.
Field identification & locations
The object appears geological and likely polished, but a definitive identification needs close-up views of surface texture, translucency, and any crystal striations, or simple hardness and streak tests. Tourmaline occurs worldwide, including Brazil, Afghanistan, and the United States.
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