igneous
Obsidian with Mahogany Phasing
Volcanic Glass (SiO2 + Fe2O3)
Hardness: 5-5.5 (Mohs); Color: Black with reddish-brown streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black with reddish-brown streaks
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Black with reddish-brown streaks; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich rhyolitic lava. The mahogany patterning is caused by the presence of hematite or magnetite inclusions (iron oxide) being swirled into the glass during volcanic flow.
Uses & applications
Used in ornamental jewelry, lapidary arts, decorative carvings, and historically for ultra-sharp surgical-grade cutting tools/blades.
Geological facts
Obsidian breaks with a conchoidal fracture, creating edges that are thinner than a molecular layer, making them sharper than steel scalpels. It is technically a 'mineraloid' because it lacks a crystal structure.
Field identification & locations
Identify by its glass-like texture, sharp edges, and unique swirled 'mahogany' patterns. Commonly found in areas of geologically recent volcanic activity such as glass buttes in Oregon, USA, or parts of Mexico.