igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide , SiO2)
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.3-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually associated with geologically recent volcanic activity from the Cenozoic era.
Uses & applications
Used in jewelry (cabochons), scalpel blades due to its ability to be honed to molecular thickness, and as a decorative stone/collector specimen.
Geological facts
Ancient civilizations used obsidian for mirrors and tools. Because it is glass, it is metastable and eventually devitrifies into crystals over millions of years, which is why it is rarely found in very old geological formations.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture which creates razor-sharp edges. It is cold to the touch and has a distinct glassy sheen when polished. Common in volcanic regions like the American West, Iceland, and Mexico.
