igneous
Obsidian
Obsidian
Hardness: 5-5.5. Color: Usually black, but can be brown, gray, or green. Luster: Vitreous (glassy). Crystal structure: Amorphous (mineraloid, no crystal structure). Cleavage: None (exhibits conchoidal fracture). Specific gravity: 2.35-2.60.
- Hardness
- 5-5
- Color
- Usually black, but can be brown, gray, or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5. Color: Usually black, but can be brown, gray, or green. Luster: Vitreous (glassy). Crystal structure: Amorphous (mineraloid, no crystal structure). Cleavage: None (exhibits conchoidal fracture). Specific gravity: 2.35-2.60.
Formation & geological history
Forms from the rapid cooling of viscous lava from volcanoes, which prevents crystal growth, resulting in volcanic glass. Typically found in areas with recent rhyolitic volcanic activity.
Uses & applications
Historically used to make cutting tools, weapons, and mirrors due to its sharp conchoidal fracture. Today, it is used in surgical scalpel blades, jewelry, ornamental carvings, and as a gemstone.
Geological facts
Obsidian blades can be much sharper than steel surgical scalpels. It is metastable at the Earth's surface and will eventually devitrify (crystallize) over millions of years, which is why ancient obsidian is rare.
Field identification & locations
Identified by its glassy luster, conchoidal fracture (smooth, curved surfaces when broken), and lack of crystalline structure. Often has sharp edges. Found worldwide in volcanic regions.
