igneous
Obsidian
Volcanic Glass
AI-generated identification · may be incorrectHardness: 5 to 5.5 on the Mohs scale; Color: Typically jet black, but can be brown, tan, or occasionally green/red due to impurities; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal fracture resulting in extremely sharp edges; Specific Gravity: Approximately 2.35 - 2.50.
- Hardness
- 5 to 5
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale; Color: Typically jet black, but can be brown, tan, or occasionally green/red due to impurities; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal fracture resulting in extremely sharp edges; Specific Gravity: Approximately 2.35 - 2.50.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It typically forms within the margins of rhyolitic lava flows.
Uses & applications
Historically used for cutting tools, arrowheads, and spear points due to its sharp conchoidal fracture. Modern uses include surgical scalpel blades, ornamental objects, and jewelry.
Geological facts
Because obsidian cools so quickly, it does not have time to form a structured crystal lattice, making it a naturally occurring volcanic glass rather than a true mineral.
Field identification & locations
Easily identifiable in the field by its intense glassy luster, dark color, and sharp conchoidal fracture edges. Found worldwide in areas with past volcanic activity, particularly in regions with rhyolitic lava flows such as the western United States, Mexico, Italy, and Japan.
