igneous
Obsidian
Volcanic Glass (Amorphous SiO2)
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically jet black, but can be dark brown, tan, or occasionally green/red, Luster: Vitreous (glassy), Crystal structure: Amorphous (lacks a crystalline structure), Cleavage: Conchoidal fracture, Specific gravity: 2.35 to 2.45
- Hardness
- 5 to 5
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Physical properties
Hardness: 5 to 5.5 on the Mohs scale, Color: Typically jet black, but can be dark brown, tan, or occasionally green/red, Luster: Vitreous (glassy), Crystal structure: Amorphous (lacks a crystalline structure), Cleavage: Conchoidal fracture, Specific gravity: 2.35 to 2.45
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcanic vent cools rapidly with minimal crystal growth.
Uses & applications
Historically used for cutting tools, arrowheads, and blades due to its extremely sharp conchoidal fracture. Also used today in jewelry, decorative objects, and occasionally in experimental surgical scalpels.
Geological facts
Because it is a glass and not a true mineral, it does not have a crystal structure. Internal arrangements of microscopic gas bubbles or crystals can sometimes create a sheen or snowflake pattern.
Field identification & locations
Easily identified by its glass-like luster, pitch-black color, and sharp conchoidal fracture edges. Commonly found in areas with past volcanic activity, such as the western United States, Mexico, and Italy.
