igneous
Polished Black Obsidian
Volcanic Glass (70-75% SiO2)
Hardness: 5-6 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
- 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; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava which prevents crystal growth. It is relatively young geologically as it eventually devitrifies into crystalline rock.
Uses & applications
Used for surgical scalpel blades, decorative jewelry, gemstones, and ornamental carvings. Historically used for arrowheads and cutting tools.
Geological facts
Obsidian is technically not a mineral because it lacks a crystalline structure. It was highly prized by the Aztecs for mirrors and weapons (the macuahuitl).
Field identification & locations
Identify by its distinctive conchoidal (shell-like) fracture and sharp edges. It is found in volcanic regions like the American West, Mexico, and Iceland.
