igneous
Obsidian
Volcanic Glass (70-75% SiO2)
Hardness: 5.0–6.0 (Mohs scale). Color: Black, deep brown, or grey (translucent at edges). Luster: Vitreous/Glassy. Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: ~2.4.
- Hardness
- 5
- Color
- Black, deep brown, or grey (translucent at edges)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5.0–6.0 (Mohs scale). Color: Black, deep brown, or grey (translucent at edges). Luster: Vitreous/Glassy. Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: ~2.4.
Formation & geological history
Formed through the rapid cooling of felsic (silica-rich) lava, which prevents crystal growth. Most specimens found today are from the Cenozoic or Mesozoic eras as silica glass devitrifies over time into more stable minerals.
Uses & applications
Historically used for arrowheads and cutting tools; currently used in jewelry (gemstone), high-end surgical scalpels for ultra-precise incisions, and architectural decoration.
Geological facts
Obsidian is so smooth it can be sharpened to a single molecule, making it sharper than even the finest steel surgical blades. It technically isn't a mineral because it lacks a crystalline structure.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and extremely sharp, glass-like edges. Common in volcanic regions such as the American Northwest, Mexico, and Iceland.
