igneous
Obsidian
Amorphous volcanic glass (70-75% SiO2)
Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has Conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- Black, sometimes dark green or 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 on Mohs scale; Color: Black, sometimes dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has Conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually found in geologically young volcanic areas.
Uses & applications
Used in gemstone jewelry, ornamental carvings, and high-quality surgical scalpel blades because it can be fractured to a microscopic edge. Historical use for arrowheads and tools.
Geological facts
Obsidian is technically not a mineral because it is amorphous and lacks a regular crystalline structure. It is relatively unstable over geological time and eventually devitrifies into crystalline minerals.
Field identification & locations
Identify by its extremely smooth, curved (conchoidal) glass-like fracture surfaces and sharp edges. Found in volcanic regions like the American West (Oregon, California), Iceland, and Mexico.
