igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or red; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Typically black but can be green, brown, or red
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or red; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Obsidian is formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is chemically rich in silica (SiO2). It is found in geologically recent volcanic areas, often from the Cenozoic era.
Uses & applications
Historically used for arrowheads and cutting tools; currently used in jewelry (gemstones), surgical scalpel blades, and decorative ornaments.
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. Ancient civilizations valued it for its extremely sharp edges, which can be thinned down to nearly a single molecule.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture, sharp edges, and glassy sheen. Commonly found in volcanic regions like the American West (Oregon, California), Iceland, Japan, and Mexico.
