igneous
Obsidian
Obsidian (Volcanic Glass, SiO2)
Hardness: 5-5.5 (Mohs scale); Color: Black, dark gray, or dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- Black, dark gray, or dark brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, dark gray, or dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of mineral crystals. It is typically found in volcanic areas of recent geological age (Tertiary and Quaternary).
Uses & applications
Historically used for edged tools like arrowheads and scalpels. Currently used in jewelry (beads/cabochons), ornamental sculpture, and high-quality surgical blades.
Geological facts
Because it is a glass and lacks a crystal structure, obsidian is chemically unstable over millions of years and eventually devitrifies into fine-grained quartz and feldspar crystals.
Field identification & locations
Identify by its glass-like texture and sharp, curved (conchoidal) fractures. Commonly found in volcanic regions like the American West, Mexico, Iceland, and Italy. Collectors look for unique variations like 'Snowflake' or 'Mahogany' obsidian.
