igneous
Obsidian
Volcanic Glass
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically black, dark brown, or green, Luster: Vitreous (glassy), Crystal structure: Amorphous (non-crystalline), Cleavage: Conchoidal fracture, Specific gravity: 2.35 to 2.50
- Hardness
- 5 to 5
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically black, dark brown, or green, Luster: Vitreous (glassy), Crystal structure: Amorphous (non-crystalline), Cleavage: Conchoidal fracture, Specific gravity: 2.35 to 2.50
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Formed in recent geological epochs associated with volcanic activity.
Uses & applications
Used historically for sharp cutting tools, arrowheads, and blades. Today used in surgical scalpels, decorative objects, and jewelry.
Geological facts
Obsidian cools so rapidly that crystals do not have time to form, resulting in its natural glass structure. It breaks with extremely sharp edges known as conchoidal fractures.
Field identification & locations
Easily identified by its glass-like appearance, dark color, and characteristic conchoidal fracture. Found worldwide in areas with past volcanic activity, particularly western North America, Mediterranean, and South America.
