igneous
Obsidian
Volcanic Glass (Amorphous Silica)
Hardness: 5-5.5 (Mohs scale); Color: Black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35-2.60.
- Hardness
- 5-5
- Color
- Black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.35-2.60.
Formation & geological history
Obsidian is an extrusive igneous rock formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in volcanic regions from the Tertiary to Quaternary geological periods.
Uses & applications
Historically used for scalpels and tools due to sharp edges; currently used for ornamental beads, jewelry, gemstones, and decorative sculptures.
Geological facts
Obsidian is technically a glass rather than a mineral because it lacks a crystalline structure. It is so sharp that some surgeons use obsidian scalpels for precision incisions because they leave less scarring than steel.
Field identification & locations
Identified by its shiny, black, glassy appearance and conchoidal (shell-like) fracture patterns. Frequently found in volcanic areas like Iceland, Mexico, and the Western United States (Oregon/California).
