igneous
Obsidian (likely black obsidian)
Volcanic glass, predominantly rhyolitic to dacitic silica-rich melt; approximate composition ~70–75% SiO₂
AI-generated identification · may be incorrectUsually jet-black, vitreous to dull, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, specific gravity ~2.35–2. R||||origin||||purpose||||facts||||significance||||value_range||||5. Black color can result from iron and other impurities.
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Physical properties
Usually jet-black, vitreous to dull, with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, specific gravity ~2.35–2. R||||origin||||purpose||||facts||||significance||||value_range||||5. Black color can result from iron and other impurities.
Formation & geological history
Forms when silica-rich lava cools extremely rapidly, preventing crystallization. Most specimens are Quaternary to relatively young, occurring around volcanic flows and domes.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads; today it is used for ornaments, carvings, jewelry, and mineral collecting, but is generally unsuitable for construction aggregate because it fractures easily.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Its razor-sharp conchoidal edges can be sharper than surgical steel, though they are brittle.
Field identification & locations
Identify by its glassy appearance, curved shell-like fractures, sharp edges, and lack of crystal grains; black slag or glass may look similar. Common localities include Mexico, the western United States, Iceland, Armenia, and Japan.
