igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
Hardness: 5-5.5; Color: Deep black, sometimes translucent at edges; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.60
- Hardness
- 5-5
- Color
- Deep black, sometimes translucent at edges
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5; Color: Deep black, sometimes translucent at edges; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.60
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. It typically originates from extrusive volcanic eruptions during the Cenozoic era or younger, as glass eventually devitrifies into crystalline rock over millions of years.
Uses & applications
Used historically for razor-sharp tools and weapons like arrowheads. Today, it is used for high-end surgical scalpels, decorative jewelry (cabochons), and ornamental carvings.
Geological facts
Obsidian fractures with edges so thin they can reach molecular levels, making it sharper than high-quality steel scalpels. In Aztec culture, it was crafted into 'Macuahuitl' clubs and used for ritual mirrors.
Field identification & locations
Identify by its smooth, glassy texture and razor-sharp curved (conchoidal) fracture lines. Commonly found in volcanic regions such as the American Northwest (Oregon), Iceland, Mexico, and the Lipari Islands. Collectors should look for 'flow' lines or 'snowflakes' (cristobalite inclusions) within the glass.
