igneous
Obsidian
Volcanic Glass (Amorphous Silica)
Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- Black to dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 Mohs; Color: Black to dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.3-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. Commonly associated with Quaternary volcanic activity in the East African Rift system.
Uses & applications
Used historically for sharp tools and weapons (scalpels, arrowheads); currently used in jewelry, ornamental carvings, and high-end surgical blades.
Geological facts
Because it is a glass and not a crystal, obsidian is thermodynamically unstable and eventually devitrifies into quartz and feldspar over millions of years. It was a major trade commodity in Neolithic times.
Field identification & locations
Identify by its characteristic conchoidal (curved/shell-like) fracture patterns and sharp edges. The location in Southern Ethiopia/Northern Kenya is consistent with volcanic deposits related to the Rift Valley.
