igneous
Obsidian
Volcanic Glass (70-75% SiO2)
Hardness: 5–6 (Mohs); Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35–2.6.
- Hardness
- 5–6 (Mohs)
- Color
- Jet black
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–6 (Mohs); Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific Gravity: 2.35–2.6.
Formation & geological history
Formed from extrusive igneous activity where high-silica lava cools extremely rapidly, preventing mineral crystals from growing. Common in young volcanic regions (Quaternary period).
Uses & applications
Used historically for sharp tools and weapons; currently used in gemstone jewelry (cabochons), ornamental carvings, and high-quality surgical scalpel blades.
Geological facts
Obsidian fractures with edges that are thinner and sharper than even high-quality steel scalpels. It is technically a 'mineraloid' rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its intense glassy sheen and characteristic curved (conchoidal) fracture lines. Commonly found in volcanic areas like the Western US (Oregon, California), Mexico, Iceland, and Japan.
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