igneous
Obsidian
Volcanic Glass (SiO2 rich)
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes grayish or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Black, sometimes grayish or brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes grayish or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed from rapid cooling of felsic lava, preventing crystal growth. Usually found in geologically young volcanic areas (Cenozoic) because the glass is unstable over long geological time and eventually devitrifies.
Uses & applications
Used in surgical scalpel blades for extreme sharpness, gemstone jewelry, ornamental sculptures, and historically for stone tools like arrowheads and knives.
Geological facts
Obsidian is often called 'nature's glass'. Because it lacks a crystal structure, its edges can be thin enough to be molecularly sharp, significantly sharper than high-quality steel scalpels.
Field identification & locations
Identify in the field by its distinct glassy luster and conchoidal 'shell-like' fractures. Commonly found in locations like Yellowstone (USA), Iceland, Mexico, and Japan. Best identified by its sharp edges and lack of visible grains.
