igneous
Obsidian
Volcanic Glass (Extrusive Igneous Rock, primarily SiO2)
Hardness: 5–6 Mohs; Color: Typically black, but can be mahogany, sheen, or snowflake; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has conchoidal fracture); Specific Gravity: 2.35–2.6.
- Hardness
- 5–6 Mohs
- Color
- Typically black, but can be mahogany, sheen, or snowflake
- 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: 5–6 Mohs; Color: Typically black, but can be mahogany, sheen, or snowflake; 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 felsic (silica-rich) lava, which prevents crystal growth. It is geologically young because it eventually devitrifies into crystalline material. Found in volcanic regions worldwide.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives). Currently used for surgical scalpel blades, jewelry (cabochons), ornamental decor, and as a semi-precious gemstone.
Geological facts
Ancient civilizations used it for mirrors and blades. Because it breaks with a conchoidal fracture, the edges can be thinner and sharper than high-quality steel, even down to a single molecular layer.
Field identification & locations
Identify in the field by its distinct glassy appearance and characteristic curved, shell-like (conchoidal) fractures. Found in volcanic fields like Glass Buttes, Oregon, or the Yellowstone area. Collectors look for unique patterns like 'Snowflake' or 'Mahogany'.
