igneous
Mahogany Obsidian
Volcanic glass (SiO2 with Hematite/Magnetite inclusions)
Hardness: 5-5.5, Color: Black with reddish-brown streaks or patches, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline glass), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5, Color: Black with reddish-brown streaks or patches, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline glass), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. The mahogany/reddish coloration is caused by inclusions of hematite or iron oxide during formation.
Uses & applications
Used in ornamental carvings, lapidary art, beads, gemstones, cabochons, and historically for sharp tools, arrowhead points, and blades.
Geological facts
Obsidian fractures with sharp edges so fine that surgical scalpels made from obsidian can have a cutting edge far thinner than high-grade steel scalpels.
Field identification & locations
Identified by its distinctive glassy luster, shell-like conchoidal fractures, and swirling mahogany-red and black bands. Commonly found in volcanic areas such as the western United States (e.g., Oregon, Glass Buttes), Mexico, and Japan.
