igneous
Obsidian
Volcanic glass
Hardness: 5 to 5.5 on the Mohs scale; Color: Deep black, sometimes dark brown, grey, or translucent; Luster: Vitreous (glassy); Crystal structure: Amorphous (lacks a true crystal structure); Cleavage: Conchoidal fracture, producing extremely sharp edges; Specific gravity: Approximately 2.35 to 2.45.
- Hardness
- 5 to 5
- Color
- Deep black, sometimes dark brown, grey, or translucent
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale; Color: Deep black, sometimes dark brown, grey, or translucent; Luster: Vitreous (glassy); Crystal structure: Amorphous (lacks a true crystal structure); Cleavage: Conchoidal fracture, producing extremely sharp edges; Specific gravity: Approximately 2.35 to 2.45.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Geological age varies greatly depending on the volcanic deposit.
Uses & applications
Historically used for making sharp tools, arrowheads, and cutting blades; currently used in jewelry, decorative ornaments, and experimentally in surgical scalpel blades.
Geological facts
Because it cools so quickly without forming a crystal lattice, obsidian is technically a mineraloid rather than a true mineral. Its conchoidal fractures can create edges sharper than high-quality steel surgical blades.
Field identification & locations
Easily identified in the field by its dark, glass-like appearance, smooth curved conchoidal fracture patterns, and razor-sharp edges. Found worldwide in areas with past volcanic activity, such as the western United States, Mexico, Italy, and Japan.
