igneous
Mahogany Obsidian
Volcanic Glass with Hematite (SiO2 + Fe2O3)
Hardness: 5-5.5 (Mohs scale); Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Fracture: Conchoidal; 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); Crystal Structure: Amorphous; Fracture: Conchoidal; Specific Gravity: 2.35-2.6
Formation & geological history
Formed when rhyolitic lava cools rapidly with minimal crystal growth, typically in volcanic regions. The red-brown streaks are caused by inclusions of Magnetite or Hematite during the cooling process.
Uses & applications
Used primarily in lapidary work for cabochons, beads, and carvings. Also used historically for sharp-edged tools and currently as a popular specimen for metaphysical collectors.
Geological facts
Unlike minerals, obsidian is a 'mineraloid' because it lacks a crystalline structure. The 'Mahogany' variety gets its name from its resemblance to the wood of the same name.
Field identification & locations
Identifiable by its glassy luster, conchoidal (shell-like) fracture patterns, and distinctive reddish-brown swirls within a black base. Commonly found in areas of recent volcanic activity such as the Western USA, Mexico, and Japan.
