igneous
Obsidian
Obsidian (Volcanic Glass, ~70% SiO2)
Hardness: 5-5.5; 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; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed from rapid cooling of felsic (silica-rich) lava, preventing crystal growth. Usually found in volcanic regions from the Quaternary period.
Uses & applications
Historical tool use (blades/arrowheads), modern surgical scalpels, jewelry (beads/cabochons), and ornamental carvings.
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. Ancient Mesoamericans like the Aztecs used it for sacrificial knives and mirrors.
Field identification & locations
Identify by its characteristic conchoidal fracture (curved, shell-like ripples) and sharp edges. Common in places like the western US, Iceland, and Mexico.
