igneous
Obsidian
Volcanic Glass (mostly SiO2)
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- Jet black to dark grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed from extrusive igneous activity when felsic lava (rich in silica) cools so rapidly that crystals do not have time to grow. Found in geologically recent volcanic fields.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives); currently used in surgical scalpels, jewelry (beads and cabochons), and as a decorative stone.
Geological facts
Obsidian is often described as a 'frozen liquid' because it lacks a crystalline structure. It breaks with extremely sharp edges—sometimes thinner than a molecule—making it sharper than steel surgical blades.
Field identification & locations
Identify in the field by its characteristic conchoidal (shell-like) fracture, glassy luster, and sharp edges. Commonly found in areas with recent volcanic activity like Oregon, California, Iceland, and Mexico.
