igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5–5.5 Mohs; Color: Jet black (sometimes brown/green); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5–5
- Color
- Jet black (sometimes brown/green)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 Mohs; Color: Jet black (sometimes brown/green); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. Usually found in geologically recent volcanic areas like the Western USA (Cascades) and Iceland.
Uses & applications
Historically used for scalpel blades, arrowheads, and tools. Modern uses include jewelry, decorative ornaments, and specialized surgical blades.
Geological facts
Because it is glass and not a crystal, it breaks with extremely sharp edges—sometimes just one molecule thick, which is sharper than high-quality steel scalpels.
Field identification & locations
Identify by its glass-like appearance and smooth, curved (conchoidal) surface breaks. Often found near rhyolitic volcanic dome sites or in riverbeds downstream from volcanic sources.
