igneous
Black obsidian
Volcanic glass (mostly rhyolitic silica glass; SiO₂-rich)
AI-generated identification · may be incorrectTypically black, opaque, and vitreous with conchoidal fracture and very sharp edges; Mohs hardness about 5–5.5, no crystal structure or cleavage, specific gravity ~2.35–2.70. The glossy surface and curved fracture strongly fit obsidian, though the image alone cannot exclude slag or another dark volcanic glass.
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Physical properties
Typically black, opaque, and vitreous with conchoidal fracture and very sharp edges; Mohs hardness about 5–5.5, no crystal structure or cleavage, specific gravity ~2.35–2.70. The glossy surface and curved fracture strongly fit obsidian, though the image alone cannot exclude slag or another dark volcanic glass.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Quaternary volcanic eruptions. Major sources include the western United States, Mexico, Iceland, Armenia, Italy, and Japan.
Uses & applications
Used for ornamental objects, cabochons, beads, knives, arrowheads, and collectible specimens; historically shaped into cutting tools because conchoidal fracture produces exceptionally sharp edges.
Geological facts
Obsidian is technically a natural glass rather than a fully crystalline rock. Snowflake, rainbow, sheen, and mahogany varieties result from crystallites, inclusions, or color zoning.
Field identification & locations
Identify by its glassy luster, smooth curved fracture, lack of visible grains, and sharp edges; it is usually lighter than iron slag and lacks bubbles or metallic flow textures. Avoid handling freshly fractured edges.
