igneous
Black polished stone, likely obsidian
Volcanic glass; primarily silica-rich SiO₂, commonly rhyolitic composition
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; opaque black with vitreous luster and conchoidal fracture. It is noncrystalline glass rather than an orderly crystal structure and has no cleavage; the apparent pale material around it may be a holder or contrasting matrix.
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Physical properties
Hardness about 5–5.5 Mohs; opaque black with vitreous luster and conchoidal fracture. It is noncrystalline glass rather than an orderly crystal structure and has no cleavage; the apparent pale material around it may be a holder or contrasting matrix.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically during Quaternary to older volcanic eruptions. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for ornamental objects, lapidary work, jewelry, tools, and historically razor-sharp blades. Value is usually modest unless the specimen has unusual color, sheen, or archaeological provenance.
Geological facts
Obsidian can produce exceptionally sharp edges because it fractures conchoidally. Varieties include snowflake, rainbow, mahogany, and sheen obsidian.
Field identification & locations
Identify it by its glassy black appearance, smooth curved fracture, sharp edges, and lack of visible grains; it should not fizz in acid. The image is low-resolution, so black flint/chert or another polished dark stone is also possible.
