igneous
Mahogany Obsidian
Volcanic Glass (SiO2 + Fe2O3)
Hardness: 5-5.5 (Mohs scale); Color: Black with reddish-brown streaks or mottling; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.60
- 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; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.60
Formation & geological history
Formed from the rapid cooling of silica-rich (rhyolitic) lava. Rapid cooling prevents crystal growth. The mahogany color comes from inclusions of Hematite or Magnetite (iron oxides) that were trapped as the lava solidified.
Uses & applications
Used primarily for jewelry (cabochons and beads), ornamental carvings, and metaphysical collecting. Historically used for making sharp-edged tools like arrowheads and knives.
Geological facts
Because it is a glass and not a mineral, it is technically a 'mineraloid'. It has been used since prehistoric times for its ability to produce an edge significantly sharper than even the highest quality steel scalpels.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture edges and smooth, glassy surface. Commonly found in volcanic regions of the United States (Oregon, California, Utah), Mexico, and Japan. Collectors should look for high contrast between the red and black patterns.
