igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-6 (Mohs); Color: Black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal; Specific Gravity: 2.35-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Black
- 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: 5-6 (Mohs); Color: Black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal; Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It occurs in areas of geologically recent volcanic activity.
Uses & applications
Used in gemstone jewelry, ornamental carvings, and historically for sharp tools and weapons. Currently used for high-precision surgical scalpels.
Geological facts
Obsidian is an extrusive volcanic glass. Because it is non-crystalline, its edges can be sharpened to nearly molecular thinness, making it sharper than even the finest steel blades.
Field identification & locations
Identify by its jet-black color, glassy surface, and shell-shaped (conchoidal) fracture patterns. Found in volcanic regions like the American Northwest, Mexico, and Iceland.
