Igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurity)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, 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 silica-rich (felsic) lava, which prevents the growth of crystals. It is typically associated with rhyolitic volcanic activity from the Cenozoic to the present, as the glass eventually devitrifies into minerals over millions of years.
Uses & applications
Used in the manufacture of extremely sharp scalpel blades, jewelry, decorative carvings, and historically for tools/weapons like arrowheads and knives.
Geological facts
Obsidians can have extremely sharp edges, thinner than the finest steel scalpel. Ancient Aztecs used obsidian for ceremonial daggers and mirrors. Some varieties show sheen like 'Rainbow' or 'Gold Sheen' obsidian due to gas bubbles or mineral inclusions.
Field identification & locations
Identify in the field by its distinct glassy appearance and shell-like (conchoidal) fractures. Found in volcanic regions like the American West (Glass Buttes), Mexico, Iceland, and Japan.
