igneous
Obsidian
Volcanic Glass (Amorphous SiO2)
Hardness: 5-6 on the Mohs scale; Color: Black, dark brown, or green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture.
- Hardness
- 5-6 on the Mohs scale
- Color
- Black, dark brown, or green
- Luster
- Vitreous (glassy)
Identified More igneous →
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Physical properties
Hardness: 5-6 on the Mohs scale; Color: Black, dark brown, or green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); 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 occasionally surgical scalpels.
Geological facts
Because it cools so quickly, no crystals can form, making it a natural glass. It was widely traded and valued in ancient civilizations for making extremely sharp cutting implements.
Field identification & locations
Easily identified by its glass-like appearance, smooth curved conchoidal fracture edges, and dark color. Common in areas with recent volcanic activity such as the western United States, Mexico, and Italy.
