igneous
Obsidian (likely black obsidian)
Volcanic glass; predominantly rhyolitic silica glass (SiO₂-rich)
AI-generated identification · may be incorrectTypically black to dark gray with vitreous luster, conchoidal fracture, no visible crystals, and hardness about 5–5.5 Mohs. The specimen appears naturally fractured or roughly knapped, with sharp edges and possible faint flow banding.
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Physical properties
Typically black to dark gray with vitreous luster, conchoidal fracture, no visible crystals, and hardness about 5–5.5 Mohs. The specimen appears naturally fractured or roughly knapped, with sharp edges and possible faint flow banding.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly during eruptions of rhyolitic volcanoes. Most specimens are Quaternary to Cenozoic in age, though obsidian occurs in older volcanic terrains as well.
Uses & applications
Used historically for exceptionally sharp blades and arrowheads; today it is collected, carved, and occasionally used in jewelry and ornamental objects. It is not generally used as a structural construction stone.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Black color commonly results from microscopic iron-bearing inclusions.
Field identification & locations
Identify by its glassy luster, lack of grains, very sharp conchoidal fracture, and inability to scratch glass reliably in every case. Common localities include Mexico, the western United States, Iceland, Armenia, and Japan.
