Igneous
Obsidian
Volcanic Glass (SiO2, usually >70% silica content)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be brown, gray, or green; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- typically jet black but can be brown, gray, or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be brown, gray, or green; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silicious lava, which prevents the growth of crystals. It is typically found in geologically recent volcanic areas (Cenozoic era) because it is metastable and eventually devitrifies into fine-grained crystals over millions of years.
Uses & applications
Historically used for making sharp weapons and cutting tools; currently used in modern surgery for high-precision scalpels, ornamental jewelry (beads, cabochons), and as a gemstone for collectors.
Geological facts
Because it has no crystal structure, obsidian fractures with extremely sharp edges—sometimes down to molecular thinness. Native Americans and other ancient civilizations used it to create arrowheads, spear points, and mirrors.
Field identification & locations
Identify in the field by its intense glassy sheen and characteristic curved 'clamshell-like' (conchoidal) fractures. Primarily found in volcanic regions of the Western USA (Oregon, California), Mexico, Iceland, and Italy (Lipari). Note: it is easily scratched by steel compared to quartz.
