igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurity isotopes)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/Glassy, Crystal 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 grey, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Formed from extrusive igneous activity when silica-rich felsic lava cools extremely rapidly, preventing crystal growth. Most specimens found today are younger than 65 million years due to the glass being metastable and devitrifying over time.
Uses & applications
Used in high-quality surgical scalpels, jewelry (rings, pendants, beads), ornamental items, and historically for ultra-sharp weapons like arrows and knives.
Geological facts
Obsidian breaks with a conchoidal fracture, creating edges that are thinner than a steel scalpel—at the molecular level. It was the basis of the 'glass sword' (macuahuitl) used by Aztec warriors.
Field identification & locations
Identify by its glass-like appearance and smooth, curved (conchoidal) breaks. Look for translucency on thin edges. Commonly found in volcanic regions like the American West (Iceland, Mexico, Japan).
