igneous
Polished black stone, likely obsidian
Volcanic glass; chiefly amorphous SiO₂
AI-generated identification · may be incorrectBlack, highly glossy vitreous luster, rounded/tumbled form, and no visible crystals or cleavage. Obsidian has Mohs hardness about 5–5.5, conchoidal fracture, and specific gravity around 2.3–2.6; surface reflections make exact identification uncertain.
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Physical properties
Black, highly glossy vitreous luster, rounded/tumbled form, and no visible crystals or cleavage. Obsidian has Mohs hardness about 5–5.5, conchoidal fracture, and specific gravity around 2.3–2.6; surface reflections make exact identification uncertain.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly in volcanic domes and lava flows, generally in Quaternary to older Cenozoic settings. The rounded shape indicates human tumbling or polishing rather than natural outcrop form.
Uses & applications
Used for decorative objects, cabochons, beads, carvings, and historically sharp tools and arrowheads. Common polished pieces typically sell for about $5–$25, with unusual sheen or patterned varieties worth more.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral. Black obsidian can resemble polished basalt, glass, or black jasper; a sharp conchoidal edge and glassy appearance support obsidian, while a streak test is not diagnostic.
Field identification & locations
Commonly found near rhyolitic volcanic regions such as Mexico, the western United States, Iceland, Armenia, and Japan. Confirm with a loupe for flow bands or gas bubbles and, ideally, hardness, density, or laboratory testing; the photograph alone cannot exclude polished basalt or slag.
