igneous
Obsidian
Volcanic Glass (SiO2 + impurities)
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3-2.6
Formation & geological history
Forms from the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is typically found in geologically recent volcanic fields or areas with rhyolitic activity.
Uses & applications
Historically used for arrowheads and cutting tools; used today for jewelry, art carvings, and high-precision surgical scalpel blades.
Geological facts
Obsidian is extremely sharp; it can produce a cutting edge that is thinner and sharper than a steel scalpel. Since it has no crystal structure, it breaks along smooth, curved surfaces.
Field identification & locations
Identify by its characteristic glassy luster and conchoidal (shell-like) fractures. It is commonly found near volcanic vents or in areas with high-silica lava flows like the western United States, Mexico, and Iceland.
