igneous
Black obsidian (likely)
Volcanic glass; primarily silica-rich, noncrystalline SiO₂ composition
AI-generated identification · may be incorrectUsually black, vitreous and highly reflective, with conchoidal fracture and sharp edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The specimen’s glassy black surface and curved fracture are consistent with obsidian, though an image alone cannot exclude slag or another dark volcanic…
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Physical properties
Usually black, vitreous and highly reflective, with conchoidal fracture and sharp edges; Mohs hardness about 5–5.5, no cleavage, and specific gravity about 2.35–2.65. The specimen’s glassy black surface and curved fracture are consistent with obsidian, though an image alone cannot exclude slag or another dark volcanic rock.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly during Quaternary volcanic eruptions. Major sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for ornamental objects, cabochons, beads, arrowheads, blades, and polished carvings; historically fashioned into exceptionally sharp tools and surgical blades.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Varieties include snowflake, rainbow, mahogany, and sheen obsidian.
Field identification & locations
Identify by its uniform glassy luster, conchoidal fracture, sharp edges, and lack of visible grains; it commonly occurs in rhyolitic lava flows and domes. A streak test is usually white or absent, and fresh edges can be razor sharp.
