igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5-6 (Mohs); Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-6 (Mohs)
- Color
- Jet black
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-6 (Mohs); Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (no crystal structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of felsic lava, which prevents the formation of crystal structures. Most obsidian is geologically young (less than 20 million years) because it eventually devitrifies into crystalline rock.
Uses & applications
Used in the manufacture of extremely sharp surgical scalpel blades, jewelry (cabochons and beads), ornamental carvings, and historically for stone tools such as arrowheads and knives.
Geological facts
Obsidian is so sharp that its edge can be thinner than a human cell. It was the basis for entire economies in ancient Mesoamerica and is popularly known in modern culture as 'Dragon Glass' from Game of Thrones.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glass-like edges. Commonly found in volcanic regions such as Oregon, California, Iceland, and Mexico. Collectors look for translucency at the edges when held to light.
