igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
Hardness: 5-5.5 on the Mohs scale. Color: Typically jet-black, but can be brown, tan, green, or translucent. Luster: Vitreous (glassy). Crystal structure: Amorphous (non-crystalline). Cleavage: Conchoidal fracture, creating extremely sharp edges. Specific gravity: 2.3-2.6.
- Hardness
- 5-5
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on the Mohs scale. Color: Typically jet-black, but can be brown, tan, green, or translucent. Luster: Vitreous (glassy). Crystal structure: Amorphous (non-crystalline). Cleavage: Conchoidal fracture, creating extremely sharp edges. Specific gravity: 2.3-2.6.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Often associated with rhyolitic eruptions.
Uses & applications
Historically used for making arrowheads, spear points, and cutting tools due to its sharp conchoidal fracture. Currently used in jewelry, ornamental items, and experimentally in surgical scalpels.
Geological facts
Because obsidian cools so quickly, it does not have time to form a structured crystal lattice, making it a natural glass rather than a true mineral. It has been used by humans for stone-age tools for hundreds of thousands of years.
Field identification & locations
Easily identified by its shiny, glass-like appearance, conchoidal fracture patterns, and razor-sharp edges. Commonly found in regions with recent volcanic activity such as the western United States, Mexico, Italy, and Japan.
