igneous
Obsidian
Volcanic Glass (predominantly SiO2)
Hardness: 5-5.5 (Mohs); Color: Jet black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black to dark brown
- Luster
- Vitreous/Glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. It occurs in geologically recent volcanic fields (often less than 10-20 million years old) due to its tendency to devitrify over time.
Uses & applications
Historically used for blade making and arrowheads; currently used in jewelry (beads/cabochons), scalpel blades for delicate surgery, and decorative ornamental carvings.
Geological facts
Because it is glass and not a mineral crystal, obsidian has no internal grain, allowing it to be fractured into blades only molecules thick, which are sharper than high-quality steel scalpels.
Field identification & locations
Identify by its glass-like appearance and distinctive curved, shell-like (conchoidal) fractures on the edges. Found in volcanic regions like the American West (Iceland, Mexico, Lipari). Collectors look for 'sheen' or inclusions like 'Snowflake' patterns.
