igneous
Black obsidian (likely)
Volcanic glass; predominantly rhyolitic, silica-rich glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectUsually black, vitreous, and noncrystalline, with conchoidal fractures and sharp edges; Mohs hardness about 5–5.5, no cleavage, specific gravity about 2.35–2.65. The pieces appear naturally fractured or tumbled and may show gray flow bands or inclusions.
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Physical properties
Usually black, vitreous, and noncrystalline, with conchoidal fractures and sharp edges; Mohs hardness about 5–5.5, no cleavage, specific gravity about 2.35–2.65. The pieces appear naturally fractured or tumbled and may show gray flow bands or inclusions.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly in volcanic domes and lava flows. Most specimens are Quaternary to older Cenozoic, though obsidian occurs in many volcanic ages.
Uses & applications
Used for ornamental objects, lapidary pieces, carvings, jewelry, and historically extremely sharp blades and projectile points; not generally used as structural stone.
Geological facts
Obsidian is technically volcanic glass rather than a true crystalline mineral. Its color may result from magnetite or other microscopic inclusions; sheen varieties reflect aligned gas bubbles or crystals.
Field identification & locations
Identify by its glassy luster, black color, very sharp conchoidal fracture, and lack of visible crystals; test cautiously because edges can cut. Common sources include Mexico, the western United States, Armenia, Iceland, Japan, and Ethiopia.
