igneous
Black obsidian (probable)
Volcanic glass; primarily SiO₂-rich rhyolitic composition
AI-generated identification · may be incorrectBlack, vitreous luster with sharp conchoidal fracture and no visible crystals; hardness about 5–5.5 Mohs, non-cleavable, specific gravity roughly 2.35–2.65. The pale, rough attached material may be pumice, rhyolite, or weathered volcanic matrix.
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Physical properties
Black, vitreous luster with sharp conchoidal fracture and no visible crystals; hardness about 5–5.5 Mohs, non-cleavable, specific gravity roughly 2.35–2.65. The pale, rough attached material may be pumice, rhyolite, or weathered volcanic matrix.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in volcanic domes and flows. Most specimens are geologically young, commonly Cenozoic, though obsidian occurs in older volcanic provinces as well.
Uses & applications
Used historically for blades and arrowheads because its fracture produces exceptionally sharp edges; today it is used for ornaments, cabochons, carvings, and geological collections.
Geological facts
Obsidian is technically a natural glass rather than a fully crystalline rock. Hydration rims and flow banding can sometimes help distinguish it from manufactured glass.
Field identification & locations
Identify by its natural irregular surface, conchoidal fracture, glassy luster, and lack of bubbles or uniform manufactured edges; common sources include Mexico, the western United States, Armenia, Turkey, and Japan. The image could also represent black glassy slag, so locality or a hardness test would improve confidence.
