igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black to dark 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 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from rapid cooling of felsic lava, which prevents the growth of crystals. Typically found in geologically young volcanic regions (Quaternary-Neogene age).
Uses & applications
Historically used for arrowheads and cutting tools; currently used in jewelry, ornamental carvings, and high-quality surgical scalpel blades.
Geological facts
Obsidian produces an edge sharper than the finest steel scalpel. Because it lacks a crystal structure, its edges can reach almost molecular thinness when fractured.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture and sharp edges. Commonly found in volcanic areas like the Western US (Oregon, California), Iceland, Japan, and Mexico.
