mineral
Polished black stone, likely black tourmaline or black obsidian
Most consistent with schorl tourmaline (NaFe3Al6(BO3)3Si6O18(OH)4), though black obsidian (volcanic glass) is also possible
AI-generated identification · may be incorrectOpaque black, vitreous to glossy luster, smoothly polished; tourmaline has Mohs hardness 7–7.5 and no cleavage, while obsidian is glassy, about 5–5.5, and conchoidally fractures. The visible pale vein-like lines may indicate tourmaline matrix or a composite/polished rough stone.
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Physical properties
Opaque black, vitreous to glossy luster, smoothly polished; tourmaline has Mohs hardness 7–7.5 and no cleavage, while obsidian is glassy, about 5–5.5, and conchoidally fractures. The visible pale vein-like lines may indicate tourmaline matrix or a composite/polished rough stone.
Formation & geological history
Black tourmaline crystallizes in boron-rich granitic pegmatites; obsidian forms when silica-rich lava cools rapidly, generally in the Quaternary to recent geologic record. A photograph alone cannot distinguish them reliably.
Uses & applications
Used as a decorative palm stone, ornamental material, and collectible; tourmaline is also used in jewelry and mineral specimens. Obsidian is used for carvings, jewelry, and historically sharp tools.
Geological facts
Schorl is the most common tourmaline variety and is strongly piezoelectric. Obsidian lacks a true crystalline structure and is technically volcanic glass.
Field identification & locations
Check edges and fractures: obsidian shows sharp conchoidal breaks, whereas tourmaline commonly reveals elongated striations or prismatic texture; hardness and streak tests can help, but avoid damaging the polished piece. Common sources include Brazil, Africa, the United States, and Mexico.
