igneous
Obsidian (Black)
Obsidian (High-silica volcanic glass)
Hardness: 5–6 (Mohs); Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6.
- Hardness
- 5–6 (Mohs)
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–6 (Mohs); Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. It is chemically similar to granite but structurally equivalent to glass. Most specimens are relatively young (Cenozoic) because the glass eventually devitrifies.
Uses & applications
Used for making scalpels (extremely sharp edges), jewelry (beads, cabochons), and ornamental carvings. Historically used for weapons like arrowheads and mirrors.
Geological facts
Despite its black color, obsidian is usually made of the same minerals as light-colored granite. When held up to a strong light, many black obsidian pebbles show translucency at the edges.
Field identification & locations
Identify in the field by its smooth, glassy texture and characteristic shell-like (conchoidal) fracture patterns. Found in volcanic regions like the American West (Glass Buttes, Oregon), Mexico, and Iceland.
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