igneous
Tumbled Obsidian (possibly Mahogany Obsidian variant)
Volcanic Glass (Amorphous Silicon Dioxide, SiO2)
Hardness: 5-6 (Mohs scale); Color: Black with potential reddish-brown (iron) inclusions; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6
- Hardness
- 5-6 (Mohs scale)
- Color
- Black with potential reddish-brown (iron) inclusions
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Black with potential reddish-brown (iron) inclusions; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.3–2.6
Formation & geological history
Formed from the rapid cooling of silica-rich rhyolitic lava, which prevents crystal growth. This process typically occurs in volcanic margins or locations where lava contacts water or air quickly. Age varies by volcanic site but is geologically young as glass devitrifies over millions of years.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, blades); modern uses include surgical scalpels, jewelry (beads, cabochons), and ornamental carvings.
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. Ancient civilizations believed it held protective powers. Even today, it is used in some specialized eye surgeries because it can be honed to a width only atoms thick.
Field identification & locations
Field identification is marked by its glassy texture and razor-sharp conchoidal (shell-like) fracturing. Commonly found in areas of recent volcanic activity like the American West (Iceland, Mexico, and the Cascades). Collectors should look for its unique smooth, curved break patterns.
