igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-6 (Mohs scale); Color: Typically black or dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-6 (Mohs scale)
- Color
- Typically black or dark brown
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Typically black or dark brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava. The cooling occurs so quickly that atoms are unable to arrange themselves into a crystalline structure. Found in geologically young volcanic regions worldwide.
Uses & applications
Historically used for arrowheads and blades; currently used in scalpels for heart and eye surgery, decorative jewelry (cabochons), and as a collector's item.
Geological facts
Because it is not a mineral (having no crystal structure), it is technically a 'mineraloid.' It breaks with a conchoidal fracture, creating edges that can be sharper than high-quality surgical steel.
Field identification & locations
Identify by its intense glassy luster and curved, shell-like (conchoidal) fracture lines. Commonly found in Oregon, California, and Iceland. Collectors should look for unique inclusions like 'snowflake' patterns or 'rainbow' sheen.
