igneous
Black obsidian (possibly industrial glass or slag)
Volcanic glass; predominantly rhyolitic composition, chiefly SiO₂
AI-generated identification · may be incorrectBlack, vitreous luster with conchoidal fracture and no visible crystal structure; Mohs hardness about 5–5.5, poor cleavage, specific gravity roughly 2.3–2.6. The rounded shape and strongly polished-looking surface may indicate a tumbled specimen or manufactured glass/slag, so identification from the image is provisiona…
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Physical properties
Black, vitreous luster with conchoidal fracture and no visible crystal structure; Mohs hardness about 5–5.5, poor cleavage, specific gravity roughly 2.3–2.6. The rounded shape and strongly polished-looking surface may indicate a tumbled specimen or manufactured glass/slag, so identification from the image is provisional.
Formation & geological history
Natural obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, mainly during Quaternary and younger volcanic eruptions. Common sources include volcanic regions of Mexico, the western United States, Armenia, Turkey, and the Andes.
Uses & applications
Used for carvings, ornaments, cabochons, arrowheads, and occasionally decorative stone; sharp flakes historically served as cutting tools. Common specimens generally have modest collector value.
Geological facts
Conchoidal fracture produces extremely sharp edges, sharper than many steel blades, though brittle. Flow bands and tiny gas bubbles can help distinguish natural obsidian from glass.
Field identification & locations
Look for curved shell-like fracture, uniform black color, glassy luster, and flow banding; test cautiously because edges can cut. Estimated value: $5–$30 for a common tumbled or rough piece, up to $50–$150 for attractive snowflake, rainbow, or high-quality large specimens.
