igneous
Obsidian
Obsidian (principally SiO2)
Hardness: 5–5.5 Mohs scale; Color: Black, dark brown, or grey (rarely green, red, or iridescent); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.35–2.6.
- Hardness
- 5–5
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs scale; Color: Black, dark brown, or grey (rarely green, red, or iridescent); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.35–2.6.
Formation & geological history
Formed from rapid cooling of felsic (silica-rich) lava, which prevents the growth of crystals. It is typically found in volcanic areas of the Cenozoic era, as it eventually devitrifies into fine-grained minerals over millions of years.
Uses & applications
Used for making surgical scalpels due to its ability to create an edge thinner than steel, ornamental jewelry, carvings, and high-end abrasive tools. Historically prized for arrowheads and scrapers.
Geological facts
Known as 'nature's glass,' obsidian can be fractured to create an edge one molecule thick. The 'Snowflake' variety contains inclusions of spherulitic cristobalite (white spots).
Field identification & locations
Identify in the field by its deep black color and distinctive conchoidal (curved, shell-like) fractures with extremely sharp edges. Commonly found in areas with geologically recent volcanic activity like Oregon, Iceland, Mexico, and Japan.
