igneous
Mahogany Obsidian
Volcanic Glass with Hematite or Magnetite (SiO2 + Fe2O3)
Hardness: 5-5.5 (Mohs scale); Color: Black with reddish-brown streaks or mottling; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black with reddish-brown streaks or mottling
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black with reddish-brown streaks or mottling; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35-2.6
Formation & geological history
Formed when rhyolitic lava cools rapidly, preventing crystal growth. The mahogany color comes from inclusions of Magnetite or Hematite during the cooling process.
Uses & applications
Used for making jewelry, decorative carvings, and scalpel blades in specialized medicine. Historically used for arrowheads and knives.
Geological facts
Despite its appearance, obsidian is a natural glass rather than a mineral because it lacks a crystalline structure. It is extremely sharp when fractured, often down to a few nanometers.
Field identification & locations
Identify by its glassy texture, shell-like (conchoidal) fracture lines, and distinctive red-brown blotches. Common in volcanic regions like Glass Buttes, Oregon, and parts of Mexico.
