igneous
Black Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. It is found in volcanic regions and can date from as recently as a few thousand years to several million years old.
Uses & applications
Used in jewelry (beads, cabochons), surgical scalpel blades (due to its ability to reach edge sharpness of molecular thickness), and decorative art. Historically used for weapons like arrowheads and spears.
Geological facts
Because it is a glass and lacks a crystal structure, obsidian is metastable and eventually 'devitrifies' over millions of years, turning into crystalline minerals. It was highly prized by the Aztecs and Mayans for both ritual and practical tools.
Field identification & locations
Identify by its high shine, jet-black appearance, and smooth, curved (conchoidal) surface when chipped. Commonly found in locations with recent volcanic activity like Mexico, the United States (Oregon, California), and Iceland.
