igneous
Tumbled obsidian
Volcanic glass; chiefly silica-rich rhyolitic glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectUsually black, glassy, and translucent at thin edges, with conchoidal fracture and no visible crystals or cleavage; Mohs hardness about 5–5.5, vitreous luster, specific gravity about 2.35–2.65. The rounded shape and polished surface indicate tumbling or deliberate shaping.
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Physical properties
Usually black, glassy, and translucent at thin edges, with conchoidal fracture and no visible crystals or cleavage; Mohs hardness about 5–5.5, vitreous luster, specific gravity about 2.35–2.65. The rounded shape and polished surface indicate tumbling or deliberate shaping.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly around Quaternary volcanic domes and lava flows. Important sources include Mexico, the western United States, Armenia, and Iceland.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and collectible specimens; historically knapped into exceptionally sharp blades and arrowheads. This piece is primarily decorative.
Geological facts
Obsidian is technically natural volcanic glass rather than a crystalline mineral. Its conchoidal fracture can produce edges sharper than surgical steel, though they are brittle.
Field identification & locations
Identify by its uniform glassy appearance, black-to-brown translucency, curved fracture surfaces, and lack of grain; unlike industrial glass, natural obsidian commonly shows flow bands or tiny inclusions. The photograph could also represent black slag or manufactured glass, so locality or testing is needed for certainty.
