Igneous
Obsidian
Volcanic glass (SiO2 - Silicon dioxide predominant composition)
Hardness: 5–6 (Mohs); Color: Jet black to dark grey; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.
- Hardness
- 5–6 (Mohs)
- Color
- Jet black to dark grey
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–6 (Mohs); Color: Jet black to dark grey; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); 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. It is primarily found in areas of geologically recent volcanic activity, dating from the Cenozoic era to present.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives); modern uses include surgical scalpels, ornamental jewelry, gemstones, and decorative home decor.
Geological facts
Because it is an amorphous glass, obsidian is metastable and over long geological periods eventually devitrifies into fine-grained mineral crystals; thus, ancient obsidian (older than Cretaceous) is rare.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and sharp edges. Commonly found in volcanic regions like the American West (Oregon, California), Iceland, and Mexico. Collectors look for translucency and surface sheen.
