igneous
Obsidian
Obsidian (Amorphous volcanic glass, predominantly SiO2)
Hardness: 5–5.5; Color: Deep black to dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Deep black to dark brown
- Luster
- Vitreous (glassy)
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5–5.5; Color: Deep black to dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed from rapid cooling of felsic lava, which prevents the growth of crystals. Typically found in volcanic regions of various ages, from the Cenozoic era to recent.
Uses & applications
Historically used for cutting tools and arrowheads; currently used in scalpels for surgery, jewelry, ornamental carvings, and gemstone collecting.
Geological facts
Obsidian can be sharper than high-quality steel scalpels. It is technically a 'mineraloid' rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its characteristic shell-like (conchoidal) fracture and sharp edges. Commonly found in places with recent volcanic activity like the western USA, Mexico, and Iceland.
