igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
Hardness: 5-5.5 Mohs; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/Glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5 Mohs; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed from felsic lava that cools so rapidly that crystals do not have time to grow. It is common in areas of recent volcanic activity (Cenozoic era/Quaternary period).
Uses & applications
Used for making scalpel blades (sharper than steel), jewelry, ornamental carvings, and historical tool-making (arrowheads/knives).
Geological facts
Obsidian is technically a 'mineraloid' because it lacks a crystal structure. Its edges can reach a thinness of nearly one molecule, making it remarkably sharp.
Field identification & locations
Look for a glassy texture and distinctive shell-like (conchoidal) fractures. Found in volcanic regions like Iceland, Mexico, and the Western United States (Oregon/California).
