igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to smoky grey, Luster: Vitreous/Glassy, 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 smoky grey, 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. Usually found in geologically young volcanic areas (Quaternary age) as it eventually devitrifies into crystals.
Uses & applications
Historically used for arrowheads and blades; currently used for surgical scalpels, jewelry (beads, cabochons), and ornamental carvings.
Geological facts
Obsidian is technically a supercooled liquid, not a mineral, because it lacks a crystalline structure. It can be sharper than a steel scalpel, cutting down to a molecular level.
Field identification & locations
Identify by its glass-like appearance and distinctive curved, shell-like (conchoidal) fracture lines. Commonly found in Oregon, Iceland, Mexico, and Lipari, Italy.
