igneous
Obsidian
Volcanic Glass (Amorphous Silica)
Hardness: 5-6 on Mohs scale. Color: Black, sometimes dark brown or green. Luster: Vitreous (glassy). Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.35-2.6.
- Hardness
- 5-6 on Mohs scale
- Color
- Black, sometimes dark brown or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-6 on Mohs scale. Color: Black, sometimes dark brown or green. Luster: Vitreous (glassy). Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.35-2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, prevents crystal growth. Usually found in geologically recent volcanic areas (Cenozoic era) because it eventually devitrifies into crystalline rock over millions of years.
Uses & applications
Historically used for arrowheads and knives. Today used in surgical scalpel blades, jewelry, decorative ornaments, and metaphysical collecting.
Geological facts
Obsidian breaks with a conchoidal fracture, producing edges that are thinner than the sharpest steel scalpels. It is not a true mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its glassy appearance, sharp edges, and smooth, curved fracture surfaces. Found in volcanic regions like the American West, Iceland, and Mexico.
