igneous
Obsidian (Black Obsidian)
Amorphous volcanic glass (primarily SiO2)
Hardness: 5.0–6.0; Color: Jet black (sometimes grey or green); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); SG: 2.3–2.6
- Hardness
- 5
- Color
- Jet black (sometimes grey or green)
- Luster
- Vitreous/Glassy
Identified More igneous →
Explore Obsidian (Black Obsidian) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5.0–6.0; Color: Jet black (sometimes grey or green); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); SG: 2.3–2.6
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in geologically recent volcanic areas (Cenozoic era).
Uses & applications
Historically used for making extremely sharp blades/arrowheads; currently used in jewelry (cabochons), surgical scalpel blades, and ornamental stone carvings.
Geological facts
Because it is a glass and lacks crystal structure, it breaks with conchoidal fractures, resulting in edges that are molecularly thin and sharper than high-quality steel scalpels.
Field identification & locations
Identify by its glass-like appearance, smooth curved (conchoidal) fractures, and jet-black color. Found in volcanic regions like the American West (Oregon, California), Iceland, and Mexico.
