igneous
Obsidian
Extrusive igneous volcanic glass (rich in SiO2)
Hardness: 5-5.5; Color: Jet black to dark grey with white crystalline inclusions; Luster: Vitreous (glassy); Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- Jet black to dark grey with white crystalline inclusions
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5; Color: Jet black to dark grey with white crystalline inclusions; Luster: Vitreous (glassy); Structure: Amorphous; Cleavage: None (conchoidal fracture); Specific gravity: 2.35-2.6.
Formation & geological history
Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. This specimen appears to be snowflake obsidian due to the presence of spherulites (white inclusions).
Uses & applications
Used in jewelry, decorative items, and historically for sharp tools and weapons. Currently used in scalpel blades for high-precision surgery.
Geological facts
Because it is glass, it is not a true mineral. It breaks with extremely sharp edges, once used by ancient cultures for arrowheads and knives.
Field identification & locations
Identified by its distinctive conchoidal (shell-like) fracture patterns and glassy texture. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland.
