igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes with banding; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline structure); Cleavage/Fracture: Conchoidal fracture with very sharp edges; Specific Gravity: 2.35-2.55.
- Hardness
- 5-5
- Color
- Black, sometimes with banding
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes with banding; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline structure); Cleavage/Fracture: Conchoidal fracture with very sharp edges; Specific Gravity: 2.35-2.55.
Formation & geological history
Formed when high-silica (rhyolitic) lava cools so rapidly that crystals do not have time to grow. It is typically found in geologically young areas with recent volcanic activity, such as the Cenozoic era.
Uses & applications
Historically used for spear points, arrowheads, and knives. Today it is used as a gemstone for jewelry, in collecting, and remarkably, for high-precision surgical scalpel blades.
Geological facts
Because it is a glass and lacks a crystalline structure, its edges can be thin enough to reach nearly molecular thickness, making it sharper than high-quality steel scalpels. It is also a primary component of 'Dragonglass' in popular culture.
Field identification & locations
Identify by its glass-like appearance, smooth curved (conchoidal) fractures, and distinct jet-black color. Commonly found in areas like the United States (western states), Mexico, Iceland, and Japan.
