igneous
Obsidian (likely)
Volcanic glass; chiefly silica-rich rhyolitic composition (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectBlack, vitreous to resinous luster with sharp conchoidal fractures and no visible crystals; hardness about 5–5.5 Mohs, amorphous structure, and poor cleavage. The brown-gray edge may indicate weathering or a thin crust.
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Physical properties
Black, vitreous to resinous luster with sharp conchoidal fractures and no visible crystals; hardness about 5–5.5 Mohs, amorphous structure, and poor cleavage. The brown-gray edge may indicate weathering or a thin crust.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically at volcanic domes, flows, and margins. It may range from recent volcanic material to specimens millions of years old, depending on locality.
Uses & applications
Used historically for razor-sharp tools and arrowheads, and today for ornaments, cabochons, decorative objects, and collectible specimens; rarely used as a major construction stone.
Geological facts
Obsidian can produce exceptionally sharp edges, sharper than many steel blades at microscopic scale. Black varieties may contain tiny gas bubbles, flow bands, or snowflake-like cristobalite inclusions.
Field identification & locations
Identify by its glassy black appearance, curved shell-like fracture, sharp edges, and lack of crystal grains; a streak test is usually absent or faint. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
