igneous
Obsidian
Obsidian (principally SiO2 silicon dioxide)
Hardness: 5-5.5 on Mohs scale; Color: typically black but can be brown, green, or mahogany; Luster: vitreous (glassy); Crystal structure: amorphous (volcanic glass); Cleavage: none (conchoidal fracture); Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- typically black but can be brown, green, or mahogany
- 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 brown, green, or mahogany; Luster: vitreous (glassy); Crystal structure: amorphous (volcanic glass); Cleavage: none (conchoidal fracture); Specific gravity: 2.35-2.6
Formation & geological history
Formed from rapid cooling of felsic (silicic) lava, which prevents the formation of crystal structures. Usually found in geologically young volcanic areas.
Uses & applications
Used for surgical scalpel blades, jewelry, decorative ornaments, and historically for arrowheads, spear points, and cutting tools.
Geological facts
Obsidian is technically a glass rather than a mineral because it lacks a crystalline structure. Ancient Aztecs used obsidian mirrors for divination, and it is still used in high-end medical surgery because it can be fractured to a thickness of only a few molecules.
Field identification & locations
Identified by its sharp, curved (conchoidal) fracture surfaces and intense glassy shine. Commonly found along the edges of rhyolitic lava flows in places like Iceland, Mexico, or the Western United States (e.g., Glass Buttes).
