igneous
Black obsidian
Volcanic glass; predominantly rhyolitic, SiO₂-rich composition
AI-generated identification · may be incorrectUsually black, vitreous, and noncrystalline, with conchoidal fracture and no cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.65. The specimen appears glossy black and naturally fractured or roughly shaped.
Identified More igneous →
Explore Black obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Usually black, vitreous, and noncrystalline, with conchoidal fracture and no cleavage; Mohs hardness about 5–5.5, specific gravity 2.35–2.65. The specimen appears glossy black and naturally fractured or roughly shaped.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, generally during Quaternary or older volcanic eruptions. Commonly associated with rhyolitic lava flows and domes.
Uses & applications
Used historically for razor-sharp tools, arrowheads, and blades; today it is collected, carved, and occasionally used in jewelry and ornamental objects.
Geological facts
Its edges can be sharper than surgical steel, but the glass is brittle and breaks easily. Small air bubbles or flow banding may produce sheen, rainbow, or snowflake varieties.
Field identification & locations
Identify by its glassy luster, smooth curved fracture surfaces, lack of visible grains, and sharp edges; it should not fizz in acid. Common localities include Mexico, the western United States, Armenia, Turkey, and Japan.
