igneous
Obsidian
Obsidian (principally SiO2, 70% or more silica)
Hardness: 5–5.5 Mohs; Color: Black to deep brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
- Hardness
- 5–5
- Color
- Black to deep brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 Mohs; Color: Black to deep brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6
Formation & geological history
Formed from extrusive volcanic activity where felsic lava cools too rapidly for crystal growth. Common in Quaternary volcanic fields (less than 2.5 million years old) as it eventually devitrifies into quartz and feldspar.
Uses & applications
Historically used for projectile points and knives. Modern uses include luxury surgical scalpels (sharper than steel), ornamental jewelry (cabochons), and landscaping stone.
Geological facts
Obsidian's conchoidal fracture creates edges that can be only molecules thick, resulting in a blade significantly sharper than high-quality steel. It was highly prized in Mesoamerican cultures for making 'macuahuitl' wooden swords with obsidian blades.
Field identification & locations
Identify in the field by its distinct glassy texture and scallop-shaped (conchoidal) fracture patterns. Frequently found in volcanic regions like the Cascades in the US, Iceland, and the Andes. Collectors should look for 'flow' patterns or 'sheen'.
