igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-6 (Mohs); Color: Black, dark gray; Luster: Vitreous; Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-6 (Mohs)
- Color
- Black, dark gray
- Luster
- Vitreous
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Physical properties
Hardness: 5-6 (Mohs); Color: Black, dark gray; Luster: Vitreous; Crystal Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in areas with recent volcanic activity (Cenozoic era).
Uses & applications
Used historically for surgical scalpels and arrowheads due to its sharp edges; used today in jewelry, decorative items, and specialized scalpels.
Geological facts
Obsidian can produce a cutting edge many times sharper than high-quality steel surgical scalpels. It is technically a mineraloid because it lacks a crystal structure.
Field identification & locations
Identify by its smooth, glassy texture and distinctive shell-like (conchoidal) fracture patterns. Found in volcanic regions like Iceland, Mexico, and the Western USA.
