igneous
Obsidian
Volcanic Glass
Hardness: 5-5.5 on the Mohs scale. Color: Typically dark black, dark brown, or grey, sometimes with sheen or iridescent colors. Luster: Vitreous (glassy). Crystal structure: Amorphous (no true crystalline structure due to rapid cooling). Cleavage: Conchoidal fracture.
- Hardness
- 5-5
- Luster
- Vitreous (glassy)
Identified More igneous →
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Physical properties
Hardness: 5-5.5 on the Mohs scale. Color: Typically dark black, dark brown, or grey, sometimes with sheen or iridescent colors. Luster: Vitreous (glassy). Crystal structure: Amorphous (no true crystalline structure due to rapid cooling). Cleavage: Conchoidal fracture.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth.
Uses & applications
Used historically for sharp cutting tools, arrowheads, and blades, as well as in modern decorative items, jewelry, and occasional surgical scalpels.
Geological facts
Because it cools so quickly, crystals do not have time to grow, resulting in a naturally occurring glass with extremely sharp edges when fractured.
Field identification & locations
Identified by its dark, glassy appearance and characteristic conchoidal fracture. Found worldwide in areas with past volcanic activity, particularly western North America, Mediterranean, and South America.
