igneous
Obsidian
Obsidian (volcanic glass; primarily SiO2)
Hardness: 5–5.5 Mohs; Color: Black, deep brown, or grey (translucent at thin edges); Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.
- Hardness
- 5–5
- Color
- Black, deep brown, or grey (translucent at thin edges)
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs; Color: Black, deep brown, or grey (translucent at thin edges); Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, preventing the growth of mineral crystals. It is typically found in volcanic regions of any geological age, though most surface samples are younger than the Cenozoic due to hydration over time.
Uses & applications
Historically used for razor-sharp tools and weapons. Modern uses include surgical scalpel blades, ornamental jewelry (cabochons), and high-end decorative objects.
Geological facts
Obsidian fractures with extremely sharp edges, often thinner than a high-quality steel blade. It is technically a 'mineraloid' rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glass-like reflection. Common in volcanic areas like Iceland, Mexico, Italy, and the western United States (e.g., Yellowstone or Glass Buttes).
