igneous
Obsidian arrowhead flakes
Volcanic glass; primarily silica-rich rhyolitic composition (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectDark gray to blue-gray, translucent on thin edges, with vitreous luster and conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and amorphous rather than crystalline structure. The sharp, deliberate flaking suggests worked obsidian artifacts or knapping debris.
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Physical properties
Dark gray to blue-gray, translucent on thin edges, with vitreous luster and conchoidal fracture; Mohs hardness about 5–5.5, no cleavage, and amorphous rather than crystalline structure. The sharp, deliberate flaking suggests worked obsidian artifacts or knapping debris.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, mainly during Quaternary and older volcanic eruptions. These pieces appear human-worked by percussion or pressure flaking, not naturally fractured.
Uses & applications
Historically used for knives, spear points, and arrowheads because its edges can be exceptionally sharp; today it is collected, studied archaeologically, and used ornamentally. Value is usually modest unless provenance, age, craftsmanship, or a documented archaeological context is established.
Geological facts
Obsidian edges can be sharper than surgical steel edges at microscopic scale. The gray-blue color may reflect composition, hydration, or lighting; chemical analysis is needed for precise source attribution.
Field identification & locations
Identify by glassy luster, lack of visible grains, curved conchoidal fracture, and razor-sharp flake scars; distinguish it from dark flint, which is usually more opaque and microcrystalline. Common sources include western North America, Mexico, Iceland, and Japan.
