Igneous
Obsidian
Volcanic Glass (SiO2 - Silicon Dioxide with impurities)
Hardness: 5-5.5 on Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Jet black; Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from felsic lava that cools so rapidly that crystals do not have time to grow. It is common in rhyolitic volcanic eruptions. Most specimens are geologically young (less than a few million years) because the glass eventually devitrifies into crystalline rock.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives); currently used in jewelry, decorative carvings, and specialty surgical scalpel blades due to its ability to produce an edge thinner than steel.
Geological facts
Obsidian is often called 'nature's glass'. While it is usually black, inclusions of magnetite or hematite can create silver or gold 'sheen' or 'rainbow' varieties. There is also a variety called 'Snowflake Obsidian' which contains white spots of cristobalite.
Field identification & locations
Identify in the field by its distinct conchoidal fracture (curved, shell-like ripples) and sharp edges. It is common in volcanic regions like the American West (Iceland, Mexico, Oregon, and the Lipari Islands). It lacks any visible granular or crystalline structure.
