igneous
Obsidian
Volcanic Glass (Amorphous Silica)
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be green, brown, or gray; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture (curved, shell-like breaks with sharp edges); Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- typically black but can be green, brown, or 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 on Mohs scale; Color: typically black but can be green, brown, or gray; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture (curved, shell-like breaks with sharp edges); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools rapidly with minimal crystal growth. It is relatively young geologically (typically less than a few million years old) as it eventually devitrifies into crystalline rock over time.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives). Today it is used in jewelry, as gemstone decorative pieces, and in high-end surgical scalpel blades because it can be fashioned to a molecularly thin edge.
Geological facts
Because it is a glass and not a crystal, obsidian does not have a grain and can be chipped in any direction to create extremely sharp edges. It was a highly traded commodity in Neolithic and Bronze Age cultures.
Field identification & locations
Identify in the field by its distinct glassy appearance and characteristic conchoidal fractures. Commonly found in volcanic regions such as the American Northwest (Oregon, California), Mexico, Iceland, and Italy.
