Igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: Black, dark brown, or grey (sometimes with iridescence); Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.
- Hardness
- 5-5
- Color
- Black, dark brown, or grey (sometimes with iridescence)
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Black, dark brown, or grey (sometimes with iridescence); Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.35–2.6.
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, which prevents the growth of mineral crystals. Typically found in volcanic areas from the Cenozoic era to the present (obsidian is geologically young as it eventually devitrifies into ordinary minerals).
Uses & applications
Historically used for making sharp tools like arrowheads and blades; currently used in jewelry (gemstones), ornamental carvings, and specialized surgical scalpels due to its incredibly sharp edge.
Geological facts
Because it has no crystal structure, its blade edges can reach almost molecular thinness, making it sharper than high-quality steel scalpels. The ancient Aztecs used and traded it extensively.
Field identification & locations
Identify in the field by its characteristic smooth, curved 'conchoidal' fractures that resemble broken bottle glass and its high shine. Commonly found in Oregon (USA), Mexico, Iceland, and regions with recent rhyolitic volcanic activity.
