igneous
Obsidian
Volcanic Glass (Rich in SiO2)
Hardness: 5-5.5 Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- Jet 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: 5-5.5 Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.3-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. It is found in volcanic regions where lava has cooled on the edges of a flow or in contact with water.
Uses & applications
Historically used for arrowheads and cutting tools; currently used in jewelry, ornamental carvings, and high-quality surgical scalpels because it can produce an edge sharper than steel.
Geological facts
Because it is glass, obsidian is unstable over long geological timeframes and eventually devitrifies into crystals; so, obsidian older than a few million years is rare.
Field identification & locations
Identify by its characteristic shell-like (conchoidal) fracture patterns, sharp translucent edges, and glassy surface. Common in Cenozoic volcanic areas like the American West (Iceland, Mexico, and Japan).
