igneous
Tumbled black stone, likely black obsidian
Obsidian (volcanic glass), primarily silica-rich rhyolitic composition
AI-generated identification · may be incorrectTypically black, very glossy vitreous luster, amorphous rather than crystalline, with no cleavage; Mohs hardness about 5–5.5 and specific gravity around 2.35–2.65. The smooth rounded surface indicates mechanical tumbling and polishing.
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Physical properties
Typically black, very glossy vitreous luster, amorphous rather than crystalline, with no cleavage; Mohs hardness about 5–5.5 and specific gravity around 2.35–2.65. The smooth rounded surface indicates mechanical tumbling and polishing.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly in volcanic domes and flows; most examples are Quaternary to relatively young geologically. The specimen was subsequently cut or tumbled by humans.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and collector pieces; naturally sharp fractures historically enabled knives and arrowheads. It has limited construction use compared with crystalline volcanic rocks.
Geological facts
Obsidian commonly shows conchoidal fracture and may contain flow bands, bubbles, or snowflake-like cristobalite inclusions. The uniformly rounded form and intense black appearance are also consistent with commercially sold tumbled obsidian.
Field identification & locations
Identify by its glassy shine, lightweight feel, lack of visible grains, and shell-like fracture on an unpolished edge; it may be confused with black glass, onyx, or tourmaline. Common sources include Mexico, the western United States, Armenia, and Japan.
