igneous
Black Obsidian
Volcanic Glass (SiO2 with impurities)
Hardness: 5-5.5 on Mohs scale. Color: Jet black to dark brown. Luster: Vitreous to shiny. Crystal Structure: Amorphous (non-crystalline glass). Cleavage: None (exhibits prominent conchoidal fracture).
- Hardness
- 5-5
- Color
- Jet black to dark brown
- Luster
- Vitreous to shiny
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Physical properties
Hardness: 5-5.5 on Mohs scale. Color: Jet black to dark brown. Luster: Vitreous to shiny. Crystal Structure: Amorphous (non-crystalline glass). Cleavage: None (exhibits prominent conchoidal fracture).
Formation & geological history
Formed when silica-rich felsic lava cools extremely rapidly with minimal crystal growth, typically near volcanic vents or flow margins.
Uses & applications
Historically used for stone tools, knapped arrowheads, and blades. Modern uses include decorative carvings, polished beads, and specialized surgical scalpel blades.
Geological facts
Because obsidian lacks a crystalline structure, its fracture edges can be sharp down to a molecular level—far sharper than high-quality surgical steel blades.
Field identification & locations
Identified by its jet-black glassy sheen and sharp, curved conchoidal fractures. Commonly found in geologically recent volcanic areas worldwide, such as Oregon, California, Iceland, and Mexico.
