igneous
Obsidian
Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)
Hardness: 5-5.5 (Mohs scale); Color: Black or deep brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black or deep brown
- 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 scale); Color: Black or deep brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. Found in geologically young volcanic areas (Cenozoic era) because it eventually devitrifies into crystalline material.
Uses & applications
Used historically for sharp tools and weapons; currently used in jewelry (cabochons), scalpel blades for delicate surgery, and as a decorative stone.
Geological facts
Obsidian is technically an igneous rock but behaves like a mineraloid. It breaks with extremely sharp edges—sometimes thinner than a human hair or a steel scalpel. A common variety called 'Apache Tears' are rounded nodules of obsidian found in the American Southwest.
Field identification & locations
Identify in the field by its characteristic conchoidal (curved, shell-like) fractures and glassy texture. Commonly found in volcanic fields such as Glass Buttes in Oregon, the Yellowstone region, and areas of Iceland and Mexico.
