igneous
Black volcanic glass (likely obsidian)
Obsidian; predominantly rhyolitic volcanic glass composed chiefly of amorphous SiO₂
AI-generated identification · may be incorrectTypically black, brown, or gray with vitreous luster, conchoidal fracture, no cleavage, and Mohs hardness about 5–5.5; the image shows a dark, irregular, glossy fragment, though its texture is partly obscured.
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Physical properties
Typically black, brown, or gray with vitreous luster, conchoidal fracture, no cleavage, and Mohs hardness about 5–5.5; the image shows a dark, irregular, glossy fragment, though its texture is partly obscured.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, usually in Quaternary to older volcanic flows, domes, and margins. Common sources include the western United States, Mexico, Iceland, Armenia, and Japan.
Uses & applications
Used historically for exceptionally sharp tools and weapons; today it is used for ornamental objects, cabochons, carvings, and lapidary study, but is generally unsuitable for structural construction.
Geological facts
Obsidian is technically volcanic glass rather than a fully crystalline rock. Hydration-rim dating can sometimes estimate its surface age, and snowflake obsidian contains cristobalite spherulites.
Field identification & locations
Identify by its glassy appearance, sharp conchoidal edges, lack of visible crystals, and usually light-colored streak. Similar black slag or glass may be artificial; natural obsidian commonly has flow banding or subtle inclusions.
