igneous
Obsidian
Obsidian (principally SiO2)
Hardness: 5.0–6.0 on Mohs scale; Color: Black (most common), can be brown, green, or banded; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline glass); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.4
- Hardness
- 5
- Color
- Black (most common), can be brown, green, or banded
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5.0–6.0 on Mohs scale; Color: Black (most common), can be brown, green, or banded; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline glass); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.4
Formation & geological history
Obsidian is an extrusive igneous rock formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in geologically recent volcanic areas, often in 'obsidian flows' or cooling edges of rhyolitic domes.
Uses & applications
Used in ornamental jewelry, decorative items, and historically for edged tools like arrowheads. In modern times, it is used to produce thin, sharp surgical scalpel blades.
Geological facts
Because it is a glass, it is metastable and eventually devitrifies (crystallizes) over time, which is why obsidian older than the Cretaceous period is rarely found. Some varieties like 'Snowflake Obsidian' contain inclusions of cristobalite.
Field identification & locations
Identify in the field by its distinct glassy luster and smooth, curved (conchoidal) fracture surfaces that resemble the inside of a seashell. Commonly found in volcanic regions of Oregon, California, Iceland, Mexico, and Japan.
