igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 (Mohs); Color: Black to dark brown with translucent edges; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves)
- Hardness
- 5-5
- Color
- Black to dark brown with translucent edges
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs); Color: Black to dark brown with translucent edges; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves)
Formation & geological history
Formed by the rapid cooling of silica-rich lava which prevents crystal growth. It is extrusive igneous rock, typically associated with volcanic activity from the Tertiary period to the present.
Uses & applications
Historically used for stone tools and arrowheads. Currently used in high-end surgical scalpels, jewelry, ornamental mirrors, and lapidary art.
Geological facts
Because it lacks a crystalline structure, obsidian is not a mineral but a mineraloid. It is sharp enough to be used in modern heart surgery because its edge is thinner and sharper than a steel scalpel.
Field identification & locations
Identify in the field by its distinctive conchoidal fracture patterns, sharp edges, and jet-black glassy appearance. It is commonly found in volcanic regions such as the American West, Iceland, and Mexico.