igneous
Likely black obsidian
Volcanic glass; chiefly rhyolitic silica-rich melt (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectBlack, opaque to slightly translucent, vitreous luster, conchoidal fracture, and no visible crystal structure; Mohs hardness about 5–5.5. The irregular angular shape and glassy surfaces are consistent with a broken or lightly weathered piece.
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Physical properties
Black, opaque to slightly translucent, vitreous luster, conchoidal fracture, and no visible crystal structure; Mohs hardness about 5–5.5. The irregular angular shape and glassy surfaces are consistent with a broken or lightly weathered piece.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly during Quaternary volcanic eruptions. Typical sources include Iceland, Mexico, the western United States, Armenia, and Japan.
Uses & applications
Used for ornamental objects, carvings, cabochons, jewelry, and historically extremely sharp cutting tools and arrowheads. It is generally unsuitable for structural construction because it fractures readily.
Geological facts
Obsidian can produce edges sharper than surgical steel but is brittle. Some varieties show sheen, rainbow colors, or snowflake patterns from microscopic inclusions.
Field identification & locations
Identify by its glassy appearance, curved shell-like fracture, sharp edges, and lack of mineral grains; a streak test is usually white or colorless. The photograph cannot exclude dark flint, basalt, or industrial glass without hardness, density, and fracture tests.
