igneous
Obsidian
Volcanic Glass (felsic composition)
Hardness: 5-5.5 on Mohs scale; Color: Typically jet black, but can be dark brown, tan, or green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture yielding extremely sharp edges; Specific gravity: ~2.35-2.46.
- Hardness
- 5-5
- Color
- Typically jet black, but can be dark brown, tan, or green
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Typically jet black, but can be dark brown, tan, or green; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture yielding extremely sharp edges; Specific gravity: ~2.35-2.46.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It typically forms along the edges of rhyolitic lava flows.
Uses & applications
Used historically for arrowheads, spear points, and cutting tools due to its sharp conchoidal fracture. Today, it is also used in decorative objects, jewelry, and occasionally in surgical scalpel blades.
Geological facts
Because it cools so quickly without forming a crystal lattice, obsidian is technically a mineraloid rather than a true mineral. Its edges can be sharper than high-tech steel surgical scalpels.
Field identification & locations
Easily identified in the field by its dark color, glassy luster, and extremely sharp conchoidal fracture edges. Found worldwide in regions with active or historic volcanic activity, particularly in western North America.
