Igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- typically jet black, can be dark green or brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed through the rapid cooling of felsic (silica-rich) lava, preventing the growth of mineral crystals. Generally found in geologically young volcanic areas (Cenozoic era) because it is metastable and eventually devitrifies into crystals.
Uses & applications
Used historically for sharp tools and weapons (arrowheads/scalpels); currently used in jewelry (cabochons), ornamental carvings, and high-end surgical blades.
Geological facts
Obsidian can be sharper than high-quality steel because it splits down to the molecular level. 'Snowflake obsidian' contains white inclusions of cristobalite, while 'Rainbow obsidian' shows colorful iridescence.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture, sharp edges, and glassy surface. Common in volcanic regions like the American West (Iceland, Mexico, Lipari). Collectors look for uniform deep black or unique inclusions.
