igneous
Obsidian
Volcanic glass (SiO2-rich)
Hardness: 5-5.5 Mohs; Color: Typically black but can be green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None, exhibits prominent conchoidal fracture.
- Hardness
- 5-5
- Color
- Typically black but can be green or 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 Mohs; Color: Typically black but can be green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None, exhibits prominent conchoidal fracture.
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Often found in geologically recent volcanic areas (Cenozoic era).
Uses & applications
Historically used for making extremely sharp cutting tools, arrowheads, and mirrors. Today used in jewelry, decorative arts, and specialized surgical scalpel blades.
Geological facts
Obsidian is technically a supercooled liquid rather than a true mineral because it lacks a crystalline structure. It is often called 'volcanic glass'.
Field identification & locations
Identify in the field by its glossy surface and razor-sharp curved (conchoidal) fracture edges. Commonly found in the western United States, Mexico, Iceland, and Japan.
