igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5; Color: Black, sometimes dark green or brown; Lustre: Vitreous (glassy); Structure: Amorphous (no crystal structure); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Black, sometimes dark green or brown
Identified More igneous →
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Physical properties
Hardness: 5-5.5; Color: Black, sometimes dark green or brown; Lustre: Vitreous (glassy); Structure: Amorphous (no crystal structure); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Obsidian forms when silica-rich felsic lava cools extremely rapidly with minimum crystal growth. It is typically geologically young because it eventually hydrates into perlite.
Uses & applications
Used historically for making sharp arrowheads and blades; currently used in scalpels for heart/eye surgery, jewelry, and various decorative ornaments.
Geological facts
Because it has no crystal structure, obsidian can be fractured to create an edge thinner and sharper than a high-quality steel scalpel. Ancient Aztecs used it for ceremonial mirrors.
Field identification & locations
Identify by its characteristic shell-like conchoidal fractures, sharp edges, and intense glassy shine. Found in volcanic regions like the American West (Glass Buttes), Iceland, and Mexico.
