igneous
Obsidian
Volcanic glass, primarily SiO₂ (noncrystalline)
AI-generated identification · may be incorrectUsually black with vitreous luster, sharp conchoidal fractures, and no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.е5. The glossy, glass-like fractured surfaces strongly fit obsidian, though the specimen may be weathered or artificially shaped.
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Physical properties
Usually black with vitreous luster, sharp conchoidal fractures, and no visible crystals or cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.е5. The glossy, glass-like fractured surfaces strongly fit obsidian, though the specimen may be weathered or artificially shaped.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically during volcanic eruptions in the Cenozoic and older periods. Common sources include the western United States, Mexico, Iceland, Armenia, Italy, and Japan.
Uses & applications
Used historically for knives, arrowheads, and mirrors; today it is collected, carved, tumbled, and occasionally used in jewelry or ornamental objects.
Geological facts
Its edges can be sharper than surgical steel, making it valuable for experimental and specialized cutting tools. Black obsidian may contain flow bands, gas bubbles, or inclusions producing sheen or “snowflake” patterns.
Field identification & locations
Identify by its uniform glassy appearance, curved shell-like fracture, sharp edges, and lack of granular crystals; scratch testing is generally unnecessary. Inspect for bubbles and flow banding, and distinguish it from industrial glass by natural flow textures and irregular volcanic form.
