igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 | Color: Typically black but can be green, brown, or red | Luster: Vitreous (glassy) | Structure: Amorphous (non-crystalline) | Cleavage: None, exhibits conchoidal fracture | Specific Gravity: 2.35-2.6
- Hardness
- 5-5
Identified More igneous →
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Physical properties
Hardness: 5-5.5 | Color: Typically black but can be green, brown, or red | Luster: Vitreous (glassy) | Structure: Amorphous (non-crystalline) | Cleavage: None, exhibits conchoidal fracture | Specific Gravity: 2.35-2.6
Formation & geological history
Formed from extrusive igneous activity when silica-rich lava cools so rapidly that crystals do not have time to grow. It is typically found in geologically recent volcanic areas.
Uses & applications
Used for making surgical scalpel blades, jewelry (cabochons and beads), ornamental carvings, and historically for sharp tools like arrowheads and knives.
Geological facts
Despite its dark color, obsidian is chemically very similar to granite and rhyolite. It is not a mineral in the strict sense because it has no orderly internal atomic structure.
Field identification & locations
Identify by its extremely smooth, glass-like texture and sharp, curved (conchoidal) edges when broken. Found in volcanic regions like the American West, Iceland, and Mexico.
