igneous
Obsidian (Polished)
Obsidian (Volcanic glass, primarily SiO2)
Hardness: 5.0–6.0 on Mohs scale; Color: Black with faint internal banding; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35–2.6.
- Hardness
- 5
- Color
- Black with faint internal banding
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5.0–6.0 on Mohs scale; Color: Black with faint internal banding; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35–2.6.
Formation & geological history
Formed through the rapid cooling of rhyolitic (silica-rich) lava, preventing crystal growth. Most specimens date from the Cenozoic era due to the fact that glass eventually devitrifies into crystalline rock.
Uses & applications
Used for surgical scalpel blades, jewelry, decorative sculptures, and home decor. Historically used by ancient cultures for arrowheads and blades.
Geological facts
Obsidian is technically a supercooled liquid because it lacks a crystal structure. It can be sharper than steel; a fractured obsidian edge is thin enough to cut through individual cells.
Field identification & locations
Identify by its glass-like texture, sharp edges when broken, and lack of visible crystals. Commonly found in volcanic regions of Mexico, Iceland, and the Western United States (e.g., Oregon, California). Collectors look for 'sheen' varieties like gold or rainbow obsidian.
