igneous
Obsidian
Volcanic Glass (Amorphous SiO2)
Hardness: 5-5.5 (Mohs); Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Obsidian forms when felsic lava extrudes from a volcano and cools so rapidly that crystal growth is inhibited, resulting in a natural glass. It is typically found in geologically young volcanic regions (Quaternary-Neogene).
Uses & applications
Used historically for arrowheads, blades, and scrapers due to its razor-sharp edges. Today it is used in scalpel blades, jewelry, ornaments, and as a collector's specimen.
Geological facts
Obsidian fractures with very sharp, curved surfaces called conchoidal fractures. Modern obsidian scalpels are up to five times sharper than steel scalpels and result in faster healing with less scarring.
Field identification & locations
Identify by its glassy texture and shell-like (conchoidal) fracture pattern. It feels smooth and cool to the touch. Commonly found in Oregon (USA), Iceland, and Mexico.
