igneous
Obsidian (volcanic glass)
Obsidian, natural silica-rich volcanic glass (typically rhyolitic; no fixed mineral formula)
AI-generated identification · may be incorrectBlack, glassy to dull on weathered surfaces, with sharp curved (conchoidal) fracture; hardness about 5–5.5 Mohs. Amorphous, with no cleavage; the specimen appears consistent with a small rough fragment.
Identified More igneous →
Explore Obsidian (volcanic glass) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Black, glassy to dull on weathered surfaces, with sharp curved (conchoidal) fracture; hardness about 5–5.5 Mohs. Amorphous, with no cleavage; the specimen appears consistent with a small rough fragment.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly at volcanic flows and domes. Usually geologically young, though obsidian can form wherever suitable lava erupts.
Uses & applications
Historically knapped into cutting tools and projectile points; today used for ornaments, jewelry, and collectible specimens.
Geological facts
Its edges can be exceptionally sharp, which is why obsidian was widely used by prehistoric toolmakers.
Field identification & locations
Look for a glassy sheen and sharp, curved fracture surfaces rather than visible grains; confirmatory identification may require examination because other dark rocks can resemble it. Found near rhyolitic volcanic deposits worldwide, including the western United States, Mexico, and Iceland.
