igneous
Obsidian (black volcanic glass)
Obsidian; primarily amorphous SiO₂-rich rhyolitic glass
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black, opaque, vitreous luster, conchoidal fracture, no cleavage, and typically ~2.3–2.6 specific gravity. The pale streaks and flow bands likely indicate rhyolitic volcanic glass with devitrified material or mineral inclusions.
Identified More igneous →
Explore Obsidian (black volcanic glass) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness about 5–5.5 Mohs; black, opaque, vitreous luster, conchoidal fracture, no cleavage, and typically ~2.3–2.6 specific gravity. The pale streaks and flow bands likely indicate rhyolitic volcanic glass with devitrified material or mineral inclusions.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, generally during Cenozoic volcanic eruptions. Commonly associated with rhyolite domes, lava flows, and volcanic ash deposits.
Uses & applications
Used for ornamental objects, cabochons, blades, tools, and jewelry; historically valued for exceptionally sharp cutting edges. Decorative specimens are usually inexpensive unless unusually patterned or translucent.
Geological facts
Its fracture can produce edges sharper than surgical steel, though they are brittle. Snowflake, mahogany, rainbow, and sheen varieties develop from inclusions or microscopic structures.
Field identification & locations
Identify by its glassy appearance, curved shell-like fracture, sharp edges, and lack of visible crystals; avoid confusing it with industrial glass or black flint. Common localities include Mexico, the western United States, Armenia, Iceland, and Japan.
