igneous
Obsidian
Volcanic Glass (Silica / SiO2)
Hardness: 5 to 5.5 on the Mohs scale; Color: Black, dark brown, or occasionally green/gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None, exhibits distinct conchoidal fracture; Specific gravity: 2.35 to 2.45.
- Hardness
- 5 to 5
- Color
- Black, dark brown, or occasionally green/gray
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale; Color: Black, dark brown, or occasionally green/gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None, exhibits distinct conchoidal fracture; Specific gravity: 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 widely depending on the volcanic deposit, ranging from recent Holocene flows to millions of years old.
Uses & applications
Historically used for making sharp cutting tools, arrowheads, and spear points. Currently used in jewelry, ornamental carvings, and occasionally in specialized surgical scalpel blades.
Geological facts
Because obsidian cools so quickly, it lacks a true crystalline structure and is classified as a mineraloid rather than a true mineral. When struck, it fractures with extremely sharp edges, known as conchoidal fracture.
Field identification & locations
Easily identified in the field by its glassy luster, dark color, and characteristic conchoidal fracture pattern. Commonly found in areas with past or present felsic volcanic activity such as the western United States, Mexico, Italy, and Iceland.
