igneous
Obsidian
Obsidian (principally SiO2)
Hardness: 5-5.5 on Mohs scale; Color: Black, dark brown, or grey; Luster: Vitreous (glassy); Crystal structure: Amorphous (glassy); Cleavage: None (conchoidal fracture); Specific gravity: 2.3 - 2.6.
- Hardness
- 5-5
- Color
- Black, dark brown, or grey
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Black, dark brown, or grey; Luster: Vitreous (glassy); Crystal structure: Amorphous (glassy); Cleavage: None (conchoidal fracture); Specific gravity: 2.3 - 2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. It is relatively young geologically, usually less than a few million years old, as it eventually devitrifies into crystal rock over time.
Uses & applications
Historically used for edged tools like arrowheads and knives; currently used in luxury jewelry, experimental surgical scalpels, and decorative ornaments.
Geological facts
Because it has no mineral crystals and breaks with a conchoidal fracture, it can be honed to an edge only a few molecules thick, making it sharper than high-quality steel scalpels.
Field identification & locations
Identifiable by its smooth, glassy texture and sharp, shell-like (conchoidal) fracture lines. It is primarily found in volcanic regions such as the American Northwest, Iceland, Mexico, and Japan.
