igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
AI-generated identification · may be incorrectHardness: 5 to 5.5 on the Mohs scale; Color: Typically jet black, but can be brown, tan, or dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35 to 2.60
- Hardness
- 5 to 5
- Color
- Typically jet black, but can be brown, tan, or dark green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale; Color: Typically jet black, but can be brown, tan, or dark green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35 to 2.60
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Formed in various geological time periods wherever volcanic activity involving silica-rich lava has occurred.
Uses & applications
Historically used for cutting tools, arrowheads, and blades due to its extremely sharp conchoidal fracture edges; currently used in jewelry, ornamental objects, and experimentally in surgical scalpels.
Geological facts
Because obsidian cools so quickly from lava, it lacks a crystalline structure, technically classifying it as a mineraloid rather than a true mineral.
Field identification & locations
Easily identified by its dark color, glassy luster, and extremely sharp, curved conchoidal fracture edges. Commonly found near recent volcanic flows worldwide, especially in western North America.
