igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be green, brown, or grayish; Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- typically jet black but can be green, brown, or grayish
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be green, brown, or grayish; Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed when silica-rich felsic lava cools rapidly with minimal crystal growth, often at the edges of basaltic lava flows or when lava touches water or air. Most specimens are relatively young geologically (Cenozoic) because the glass eventually devitrifies into crystalline rock.
Uses & applications
Used historically for sharp cutting tools and weapons (arrowheads, scalpels). Currently used in jewelry, ornamental carvings, and high-end surgical blades for its ability to hold an edge thinner than steel.
Geological facts
Because it is a glass and not a true mineral, it lacks a crystal structure. Apache Tears are a popular variety of rounded, translucent obsidian nodules. It can produce a cutting edge so sharp it can slice through individual cells.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture, sharp edges, and glassy luster. Commonly found in volcanic regions such as the American Northwest (Oregon, California), Iceland, Japan, and Mexico. Collectors should look for smooth, glassy surfaces that lack any visible grain.
