igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2
Hardness: 5–5.5 (Mohs scale); Color: Black, dark green, or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6
- Hardness
- 5–5
- Color
- Black, dark green, or brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 (Mohs scale); Color: Black, dark green, or brown; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6
Formation & geological history
Formed from extrusive igneous activity when felsic lava cools so rapidly that crystals do not have time to grow. It is common in geologically recent volcanic areas, often from the Quaternary period.
Uses & applications
Used historically for sharp tools and weapons (scalpels, arrowheads). Modern uses include jewelry, decorative carvings, and as a gemstone for collectors.
Geological facts
Because it doesn't have a crystal structure, obsidian fractures into extremely sharp edges, leading to its historic use in surgery and hunting. Unlike most rocks, it is technically a 'mineraloid'.
Field identification & locations
Identify in the field by its high-gloss vitreous shine and smooth, curved (conchoidal) fracture surfaces. Commonly found in Iceland, Mexico, the United States (Oregon/California), and Japan.
