igneous
Obsidian
Volcanic Glass (mostly SiO2)
Hardness: 5-5.5; Color: Jet black, sometimes translucent brown on thin edges; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black, sometimes translucent brown on thin edges
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5; Color: Jet black, sometimes translucent brown on thin edges; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of felsic lava, which prevents crystal growth. It is usually found in geologically young volcanic areas (Cenozoic era) because it eventually devitrifies into crystals.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives); modern uses include surgical scalpels, ornamental jewelry, and decorative gemstones.
Geological facts
Because it breaks with a conchoidal fracture, it produces edges thinner and sharper than a steel scalpel. Native Americans traded obsidian over hundreds of miles throughout North America.
Field identification & locations
Identify in the field by its glassy texture and razor-sharp curved edges when broken. Commonly found in volcanic regions of Oregon, California, Iceland, and Mexico.
