igneous
Obsidian
Volcanic Glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5–6 on Mohs scale; Color: typically black but can be dark green, brown, or grayish; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
- Hardness
- 5–6 on Mohs scale
- Color
- typically black but can be dark green, brown, or grayish
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–6 on Mohs scale; Color: typically black but can be dark green, brown, or grayish; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
Formation & geological history
Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in many locations worldwide that have experienced rhyolitic eruptions, primarily during the Cenozoic era.
Uses & applications
Historically used for making sharp tools and weapons (arrowheads, blades). Today used in jewelry, as ornamental stones, and in specialized surgical scalpels due to its incredibly sharp edge.
Geological facts
Because it is a glass and lacks a crystal structure, obsidian is technically naturally occurring volcanic glass rather than a true mineral. It is often sharper than high-quality surgical steel on a microscopic level.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and extremely smooth, glassy surfaces. Common locations include the Western United States (Oregon, California), Mexico, Iceland, and Italy. Collectors look for unique varieties like Snowflake or Mahogany Obsidian.
