igneous
Black obsidian arrowhead
Obsidian; volcanic glass, typically rhyolitic in composition (mostly SiO₂)
AI-generated identification · may be incorrectBlack, vitreous luster, conchoidal fracture, no visible crystal structure, and Mohs hardness about 5–5.5; the flaked, sharp edges indicate intentional knapping. Specific gravity is approximately 2.3–2.5.
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Physical properties
Black, vitreous luster, conchoidal fracture, no visible crystal structure, and Mohs hardness about 5–5.5; the flaked, sharp edges indicate intentional knapping. Specific gravity is approximately 2.3–2.5.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary to older volcanic eruptions. The specimen appears human-shaped or knapped from obsidian rather than naturally fractured.
Uses & applications
Used historically for cutting tools, projectile points, and ceremonial objects because its conchoidal fracture produces extremely sharp edges; today it is mainly a collectible or archaeological artifact.
Geological facts
Obsidian is technically volcanic glass rather than a crystalline mineral. Its razor edges can be sharper than surgical steel, though they are brittle and easily chipped.
Field identification & locations
Identify by glassy black appearance, curved fracture surfaces, lack of grain, and flake scars; common sources include Mexico, the western United States, Armenia, Italy, and Japan. Archaeological context is important for determining authenticity and age.
