igneous
Mahogany Obsidian
Volcanic glass containing Hematite or Magnetite (SiO2 + Fe2O3)
Hardness: 5-5.5 on Mohs scale; Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Cleavage: None (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 on Mohs scale; Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from the rapid cooling of rhyolitic lava which prevents crystallization. The mahogany color is produced by high concentrations of iron oxides like hematite or magnetite trapped within the glass. Found in volcanic areas with high-silica eruptions, often dating from the Tertiary to Quaternary periods.
Uses & applications
Commonly used in jewelry (beads, cabochons), ornamental carvings, and metaphysical practices. Historically used to make sharp cutting tools and arrowheads.
Geological facts
Obsidian is technically not a mineral but a 'mineraloid' because it lacks a crystalline structure. The reddish-brown 'mahogany' patterns are often flow-banded, representing the movement of the lava as it cooled.
Field identification & locations
Identify by its glassy texture and distinctive reddish-brown swirls against a black background. It always exhibits conchoidal (shell-like) fracturing. Primarily found in the western United States (Oregon, California, Utah), Mexico, and volcanic regions of Japan.
