igneous
Mahogany Obsidian
Volcanic Glass (predominantly SiO2 with Fe2O3 inclusions)
Hardness: 5–5.5 Mohs scale; Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); 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 Mohs scale; Color: Black with reddish-brown streaks or mottling; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
Formation & geological history
Formed through the rapid cooling of felsic (rhyolitic) lava, which prevents crystal growth. The mahogany coloration is caused by inclusions of hematite or magnetite (iron oxides) within the volcanic glass.
Uses & applications
Used in the lapidary arts for gemstones, beads, and carvings. Historically used for making sharp tools like arrowheads and knives. Also popular in metaphysical and collecting circles.
Geological facts
Obsidians are technically 'mineraloids' rather than minerals because they lack a crystalline structure. Mahogany obsidian is chemically unstable over millions of years, eventually 'devitrifying' into a stony rock.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glassy texture. Commonly found in areas of recent volcanic activity such as the Western United States (Oregon, California), Mexico, and Japan.
