igneous
Obsidian (likely mahogany or smoky obsidian)
Volcanic glass, predominantly rhyolitic silica (SiO₂-rich), noncrystalline
AI-generated identification · may be incorrectBlack to dark brown, vitreous/glassy luster, conchoidal fracture, no cleavage, and typically Mohs hardness 5–5.5; the translucent gray-brown patches and rusty inclusions may reflect devitrification or iron oxides.
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Physical properties
Black to dark brown, vitreous/glassy luster, conchoidal fracture, no cleavage, and typically Mohs hardness 5–5.5; the translucent gray-brown patches and rusty inclusions may reflect devitrification or iron oxides.
Formation & geological history
Forms when silica-rich lava cools too rapidly for crystals to grow, commonly during Quaternary volcanic eruptions; exact age and locality cannot be determined from the image.
Uses & applications
Used for ornamental objects, cabochons, lapidary work, and historically extremely sharp blades and arrowheads. Common material generally has modest collector value.
Geological facts
Its curved, shell-like fracture can produce edges sharper than surgical steel. The piece appears naturally rough with some polished or freshly fractured reflective surfaces.
Field identification & locations
Identify by glassy appearance, sharp conchoidal edges, lack of visible crystal grains, and absence of cleavage; test cautiously because edges are sharp. Common deposits occur in volcanic regions such as the western United States, Mexico, Armenia, Iceland, and Italy.
