igneous
Black Obsidian
Volcanic Glass (SiO2 + impurities)
Hardness: 5–5.5 Mohs scale; Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Jet black
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5–5.5 Mohs scale; Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. Typically found in volcanic regions from the Holocene to Tertiary periods.
Uses & applications
Used in the manufacture of surgical scalpel blades, jewelry (cabochons and beads), ornamental carvings, and historical stone tools (arrowheads).
Geological facts
Obsidian is technically not a mineral because it is a glass and does not have a crystalline structure. Native Americans used it for high-precision tools because it can be fractured to an edge thinner than a steel scalpel.
Field identification & locations
Identify by its characteristic conchoidal fracture (curved, shell-like edges) and glassy luster. Commonly found in Oregon, California, Iceland, and Mexico. Collectors look for translucency on thin edges.
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