Igneous
Black Obsidian
Volcanic Glass (Amorphous Silicon Dioxide SiO2)
Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark gray; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black to dark gray
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark gray; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from rapid cooling of felsic (silicic) lava, preventing crystal growth. Usually found in margin areas of rhyolitic lava flows. Geological age varies by location, typically younger than a few million years as it eventually devitrifies.
Uses & applications
Used for making surgical scalpel blades (sharper than steel), ornamental jewelry, decorative carvings, and meditation stones. Historically used for arrowheads and scrapers.
Geological facts
Obsidian fractures with extremely sharp edges, even down to molecular thickness. It was highly prized by the Aztecs for making 'macuahuitl' (club-swords) and mirrors.
Field identification & locations
Identify by its glass-like appearance and characteristic conchoidal (shell-like) fracture patterns. Commonly found in volcanic regions like the American West (Oregon, California), Mexico, Iceland, and Italy. Often found as rounded 'Apache Tears'.
