igneous
Obsidian
Volcanic Glass (Amorphous Silica, SiO2)
Hardness: 5-5.5 Mohs; Color: Typically black but can be green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Typically black but can be green or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 Mohs; Color: Typically black but can be green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed when extrusive felsic lava cools extremely rapidly with minimum crystal growth. Common in areas with recent volcanic activity like the western United States, Mexico, and Iceland.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, blades). Today used in jewelry, decorative ornaments, and specialized surgical scalpel blades.
Geological facts
Obsidian is technically not a mineral because it is amorphous glass and lacks a crystal structure. It was highly prized by ancient civilizations like the Aztecs and Mayans.
Field identification & locations
Identify by its razor-sharp edges and smooth, curved (conchoidal) fracture surfaces that look like broken glass. Usually found in volcanic lava flows.
