igneous
Obsidian
Volcanic glass (high silica content based on SiO2)
Hardness: 5-5.5 (Mohs); Color: Raven black, often translucent on thin edges; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Raven black, often translucent on thin edges
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Raven black, often translucent on thin edges; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Most specimens belong to the Cenozoic era, as volcanic glass eventually devitrifies into quartz and feldspar over millions of years.
Uses & applications
Used in the manufacture of surgical scalpels (blades are thinner than steel), jewelry/cabling, and various decorative items. Historically vital for making arrowheads and cutting tools.
Geological facts
Obsidian is so sharp at the molecular level that it can be used for heart surgery to minimize scarring. Unlike most rocks, it is technically an 'extra-viscous liquid' rather than a true solid due to its lack of internal crystalline structure.
Field identification & locations
Identify by its distinctive conchoidal fracture (curved, shell-like breaks) and razor-sharp edges. Common in volcanic regions with high silica rhyolitic flows, such as Oregon, California, Iceland, and Mexico.
