igneous
Obsidian
Volcanic Glass (70% or more SiO2)
Hardness: 5.0–6.0 on Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3–2.6.
- Hardness
- 5
- Color
- Jet black
- Luster
- Vitreous (glassy)
Identified More igneous →
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Physical properties
Hardness: 5.0–6.0 on Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3–2.6.
Formation & geological history
Formed from rapid cooling of felsic (silica-rich) lava, preventing crystal growth. Usually found in volcanic regions of relatively young geological age (Cenozoic) because it eventually devitrifies into quartz and feldspar.
Uses & applications
Historically used for cutting tools and arrowheads; currently used in jewelry (polished stones), ornamental carvings, and specialized surgical scalpel blades.
Geological facts
Obsidian was highly prized in Mesoamerica for mirrors and sacrificial blades. Because it breaks with a conchoidal fracture, the edges can be thinner and sharper than high-quality steel scalpels.
Field identification & locations
Identify in the field by its characteristic glassy sheen and sharp, shell-like (conchoidal) edges when broken. Commonly found in volcanic areas like the Western USA, Iceland, and Mexico.
