igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 | Color: Typically black, but can be green, brown, or striped | Luster: Vitreous/Glassy | Structure: Amorphous | Cleavage: None, exhibits conchoidal fracture | Specific Gravity: 2.3-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 | Color: Typically black, but can be green, brown, or striped | Luster: Vitreous/Glassy | Structure: Amorphous | Cleavage: None, exhibits conchoidal fracture | Specific Gravity: 2.3-2.6
Formation & geological history
Extrusive igneous rock formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in geologically young volcanic areas due to its instability over geological time.
Uses & applications
Used for making exceptionally sharp surgical blades, ornamental jewelry (cabochons), gaming dice, and decorative carvings. Historically used for arrowheads and knives.
Geological facts
Obsidian is so sharp that its edge is sometimes thinner than a human cell. It is 'mineral-like' but not a true mineral because it lacks a crystalline structure.
Field identification & locations
Identified by its smooth, curved conchoidal fractures and sharp edges. Commonly found in volcanic regions like the Cascades (USA), Iceland, and Mexico. Collectors should look for 'flow' patterns or 'sheen' varieties.
