igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-6 (Mohs); Color: Black with grey or white flow banding; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Black with grey or white flow banding
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-6 (Mohs); Color: Black with grey or white flow banding; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (rhyolitic) lava, preventing crystal growth. It is relatively young geologically (typically Cenozoic) because it eventually devitrifies into quartz and feldspar over millions of years.
Uses & applications
Used for making scalpels, jewelry, semi-precious gemstones, and ornamental carvings. Historically used for arrowheads and blades.
Geological facts
Obsidian is often called 'nature's glass'. The greyish-white banding seen in this specimen is known as flow banding, caused by the stretching of inclusions as the lava flowed before solidifying.
Field identification & locations
Identify in the field by its distinctive conchoidal (shell-like) fracture, extremely sharp edges, and glassy texture. Look for it in volcanic regions like the American West (Glass Buttes, OR).
