igneous
Obsidian
Volcanic Glass (Silicon Dioxide with impurities)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark brown/green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark brown/green, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is typically found in quaternary volcanic regions associated with subduction zones or hotspots.
Uses & applications
Used historically for arrowheads and knives; currently used for surgical scalpel blades, ornamental jewelry like beads and cabochons, and as a collector's specimen.
Geological facts
Because it is glass, obsidian breaks with a conchoidal fracture, creating edges that are thinner and sharper than high-quality steel surgical scalpels. It is technically a mineraloid rather than a mineral because it lacks a crystal structure.
Field identification & locations
Identify in the field by its distinct glassy appearance, smooth curved (conchoidal) fracture lines, and sharp edges. Common in volcanic areas like Iceland, Mexico, and the Western USA (Oregon, California). Collectors look for variations like 'Snowflake' or 'Mahogany' obsidian.
