igneous
Obsidian
Volcanic glass (high silica SiO2)
Hardness: 5–5.5 Mohs scale; Color: Black, sometimes with dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Black, sometimes with dark green or brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs scale; Color: Black, sometimes with dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.4
Formation & geological history
Formed from extrusive igneous activity when high-silica rhyolitic lava cools rapidly, preventing crystal growth. It is found in geologically young volcanic areas usually dating from the Pliocene to the Holocene.
Uses & applications
Historically used for arrowheads and blades; currently used in high-end surgical scalpels, lapidary art, jewelry, and ornamental decor.
Geological facts
Because it is an amorphous glass and lacks crystalline structure, obsidian is not technically a mineral. It was highly prized in Mesoamerican cultures for making 'Macuahuitl' (wooden swords with obsidian blades).
Field identification & locations
Identify by its characteristic glassy surface and smooth, curved conchoidal fractures. Common in volcanic regions such as the American Northwest (Oregon/California), Mexico, Iceland, and Italy (Lipari).
