igneous
Obsidian
Volcanic Glass (SiO2 + impurities)
Hardness: 5-5.5 on Mohs scale. Color: Black to dark grey. Luster: Vitreous/glassy. Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.4.
- Hardness
- 5-5
- Color
- Black to dark grey
- Luster
- Vitreous/glassy
Identified More igneous →
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Physical properties
Hardness: 5-5.5 on Mohs scale. Color: Black to dark grey. Luster: Vitreous/glassy. Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.4.
Formation & geological history
Formed through the rapid cooling of silica-rich lava (rhyolitic) which prevents crystal growth. Usually found in geologically young volcanic regions (Quaternary period).
Uses & applications
Historically used for arrowheads and cutting tools. Today used in scalpel blades, jewelry, and as decorative gemstones.
Geological facts
Because it is glass and lacks a crystal structure, its edges can be thinned down to nearly molecular thickness, making it sharper than even the finest steel surgical scalpels.
Field identification & locations
Identify by its glass-like appearance and conchoidal (shell-like) fracture patterns. Frequently found in volcanic fields such as those in the Western United States, Mexico, and Iceland.
