igneous
Black Obsidian
Volcanic Glass (70–75% SiO2 plus MgO, Fe3O4)
Hardness: 5–5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5–5
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed when felsic lava extrudes from a volcano and cools rapidly with minimal crystal growth. Common in areas of geologically recent volcanic activity.
Uses & applications
Used primarily in jewelry (beads, cabochons), ornamental carvings, and high-precision surgical scalpel blades due to its ability to produce an edge thinner than steel.
Geological facts
Obsidian was highly prized by ancient civilizations like the Aztecs and Mayans for making extremely sharp weapons like the macuahuitl and sacrificial daggers.
Field identification & locations
Identify by its glass-like appearance, jet black opacity, and sharp conchoidal (shell-like) fractures. Common in volcanic regions like Iceland, Mexico, and the Western United States.
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