igneous
Black obsidian
Volcanic glass, primarily silica-rich rhyolitic composition (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectBlack, strongly glassy/vitreous luster with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, and specific gravity around 2.3–2.6. The blue reflective flashes may indicate sheen obsidian or surface iridescence.
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Physical properties
Black, strongly glassy/vitreous luster with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, poor cleavage, and specific gravity around 2.3–2.6. The blue reflective flashes may indicate sheen obsidian or surface iridescence.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, usually at volcanic domes and lava flows. Most specimens are Quaternary to Cenozoic in age, though obsidian occurs in older volcanic terrains.
Uses & applications
Used for ornamental objects, jewelry, cabochons, tools, and historically razor-sharp blades and arrowheads; not generally used as structural aggregate.
Geological facts
Its conchoidal fracture can produce edges sharper than surgical steel, although they are brittle. Blue sheen commonly results from microscopic mineral or flow-related structures that reflect light.
Field identification & locations
Identify by its uniform glassy appearance, sharp curved fracture surfaces, and lack of grain; test cautiously because edges are extremely sharp. Common sources include Mexico, the western United States, Armenia, Ethiopia, and Iceland.
