igneous
Obsidian
Amorphous Volcanic Glass (primarily SiO2)
Hardness: 5-5.5 (Mohs); Color: Jet black to dark gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black to dark gray
- Luster
- Vitreous (glassy)
Identified More igneous →
Explore Obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black to dark gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
Formation & geological history
Formed from rapid cooling of felsic (silica-rich) lava, preventing crystal growth. Usually found in volcanic margins and dated from the Cenozoic era to recent times.
Uses & applications
Used in gemstone manufacturing, knapping for surgical scalpels, decorative ornaments, and abrasive grit. Highly valued in historical toolmaking.
Geological facts
Because it is chemically unstable over long geological time periods, obsidian older than a few million years is rare, as it eventually devitrifies into crystalline rock.
Field identification & locations
Identify by its sharp edges and smooth, curved (conchoidal) fracture surfaces like broken glass. Commonly found in Oregon, Iceland, and Mexico. Collectors should look for unique internal patterns like 'snowflake' or 'rainbow' sheen.
