igneous
Obsidian
Obsidian (Volcanic Glass)
Mohs hardness: 5-6; Color: Black, dark green, or brown; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.6.
- Hardness
- 5-6
- Color
- Black, dark green, or brown
- Luster
- Vitreous (glassy)
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Physical properties
Mohs hardness: 5-6; Color: Black, dark green, or brown; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed by the rapid cooling of silica-rich (rhyolitic) lava, preventing crystal growth. It is relatively young geologically as it eventually devitrifies into crystalline rock.
Uses & applications
Used for making sharp blades, surgical scalpels, jewelry (beads/cabochons), and ornamental carvings.
Geological facts
Because it lacks a crystalline structure, obsidian fractures with razor-sharp edges that are thinner and sharper than high-quality steel scalpels. Ancient cultures used it for weapons like arrowheads and spears.
Field identification & locations
Identified in the field by its distinctive glassy appearance and conchoidal fracture patterns. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland.
